Aggregators

Aggregators are search engines or booking agencies that combine information about many hotels, transportation providers or the like.

Understand

Most of the time you can book hotels, flights, buses, trains, cruises and car rentals directly through the aggregator (who may receive a commission for your booking) but there are also those that only list prices and direct you to other pages for booking. While aggregators are not strictly essential and goods or services booked through them can be more expensive than those booked directly, they can be a godsend in weeding through countless offers for example in air travel.

For hotels, cruises, etc., only the major players may be part of the comparison. Thus, especially for off the beaten track destinations, you might want to use other means to find local businesses. If an aggregator says there are no options available, you might still want to keep looking—it could be they just don't have complete or up-to-date information for the places and dates you're looking at.

Aggregators are notorious for displaying different prices to different customers based on browsing history, past purchases, and other inscrutable factors. It may help to clear your cookies and cache, or to open a "private" window in your browser before using these websites.

Aggregators generally aren't linked on other Wikivoyage pages, which aim to list transport operators or hotels directly.

Cashback

Various goods and services when booked online can be gotten with a certain "cashback": a website gets a cut for every sale they enable and they hand on a part of that cut to its users. Cookies from an aggregator search can cause the cashback to be rejected, so surf with caution.

Missed connections

If all legs of the trip are with the same company or alliance, they usually take responsibility to rebook you onto the next flight/train/bus in case of a delay that lets you miss the originally planned connection. When you book the legs from different companies, you are often on your own.

Some aggregators explicitly guarantee their connections (even if booked on different companies) when booked through them. This may be the only way to ensure you'll be rebooked, avoiding the hassle and costs of having to book hotels and new flights yourself, when stranded at the airport. By no means all aggregators provide that guarantee, and even when they do there may be caveats like "hand luggage only". Read the fine print. If no such guarantee is given, the territory gets very murky.

Air travel aggregators

Nowadays air travel is only rarely booked directly through the airline without first searching and comparing prices. Sometimes the same flight can have vastly differing prices at various agregators and it pays to compare search results and to also look at the website of the airline itself before booking.

Rail air alliances, especially rail&fly are frequently difficult or impossible to book through aggregators and the vast majority of them won't know IATA codes for railway stations or rail&fly as a whole (QYG). You might have better luck directly with the airline, but sometimes the easiest way to book stuff like this is to actually go talk to an actual human. Given that rail&fly must be booked with your ticket for most flights (some airlines allow it to be added later on, but not even all airline staff know that) and offers both a better price and more flexibility than booking a train trip separately, this is no small thing, especially when you know the price difference to be considerable. Another frequent annoyance with air travel aggregators is that they often want to upsell or offer you hotels, rental cars and the like that you might not need or want all things considered or that can be gotten cheaper through other sources. Aggregators are also known to use cookies to see which flight routes you want to buy, charging you more for a flight you desperately want to take: clear cookies, use private or incognito browsing or search from a different browser or computer to diminish this issue.

If you are flexible with your dates, some aggregators allow searches for +/- three days and others display prices for different departure dates, but you'd have to run the search again to see how accurate those are. Some airlines, most notoriously Ryanair and Southwest, may not show up in aggregator searches, so it might pay to compare with those as well.

Kayak — a popular aggregator, good at finding cheap flights, with lots of options to modify your search.
Momondo — reliably finds the cheapest (or close to the cheapest) option available. Good user interface. Allows you to search by frequent flier alliance if you're looking to accumulate miles.
Google Flights — fast but doesn't always find the cheapest options. Has features that show you the best dates to fly if your dates are flexible and the cost of different destinations if you haven't yet decided where to go.
Skyscanner — includes some airlines that don't usually appear in aggregators, and allows you to set up to 30 "watchdogs" to search for unusually cheap fares (often errors by airlines, which can allow you to travel very cheaply if you act fast). Also lets you search without specifying a destination so you can see what destinations are cheapest for the chosen dates. "Whole month" feature does not usually work on secondary destinations.
Flightnetwork — pairs several low cost airlines that normally other aggregators do not pair making it a great option for international routes.
Kiwi.com — allows search from multiple airports to multiple airports, on flexible dates. Sells tickets at higher prices, but it is possible to just find the flight and buy it elsewhere
Tripchemy — fast and cheap multi-city round trip search engine for destinations in or around Europe. You buy the tickets yourself using Google Flights. Great if you want to visit multiple destinations on a budget.
CapeTowncheapflights — global flight comparison and travel website, which uses 728 airlines and some travel agencies to list prices and direct you to appropriate booking pages.
Flights50off- Flights aggregator showing results from different OTAs in seconds.

To find a low-cost/no-frills flight it can be good to check one of the comparison tools, such as, e.g., flylowcostairlines.org.

Sites such as Expedia and Travelocity can help you explore your options, but these may not show budget airline flights, and they are rather North America-centric, often showing ridiculously inflated (full-fare) prices for travel outside North America.

Train aggregators

In general booking through the railway directly will get you the best fares, but in some cases there are different railways to compare and tickets across several countries can be challenging to book online.

Bus aggregators

Overall, the bus market is still largely separate for separate countries, in part due to the large travel times involved in crossing any significant distance.

The aggregators usually cover only part of the market and may have deficient name engines, so "no connection" may mean that the company providing the service has no agreement with the aggregator, or that the server did not understand that a bus with a stop on the thoroughfare instead of at the bus station is good enough for you.

International

Andestransit.com covers all countries in South America. Worth a look along with other sites, but leaves something to be desired in terms of accuracy and completeness.
Busbud covers most operators in the US, and operators in 62 other countries
Busradar now covers many parts of Europe, with varying level of completeness. See Germany below.
busticket4.me covers 27 European countries
Check My Bus searches for buses in 47 countries
RedBus covers Colombia, Peru, India, Indonesia, Malaysia and Singapore in separate portals.

Bolivia

Tickets Bolivia for buses and trains

Brazil

Busca Ônibus

Cambodia

Camboticket

Central America

Horarios de Buses - schedules only, but there are no prebooking discounts for local buses anyway.

Central Europe

AMSBUS - sells tickets for Czechia and Slovakia and European routes. Popular timeteble finder IDOS redirects here

Finland

Matkahuolto sells tickets and maintains a route planner for nearly all non-local bus services in Finland, since way before the web. It also maintains the network of bus stations and agents substituting bus stations.
Perille.fi combines bus and train connections, has also domestic flights
Matkakeisari.fi - bus and train connections
Pikavuorot.fi - bus and train connections

Germany

Netherlands

9292 allows you to check and search for your connections by public transit (trains, buses, trams and metros are covered) and gives you an easy overview of the quickest, easiest, earliest and cheapest connections to your destination. The service can be taken on-the-go when offline via their app for iOS and Android.

United Kingdom

Coach Fairer

USA

GoToBus

Taxi aggregators

Ride.guru - online tool that calculates potential fares for taxis, Uber, Lyft, Didi, Bolt, Ola etc.
Bellhop - app for Android and iOS, compares Uber and Lyft, it also provides data from other ride-hailing companies such as Juno, Curb, and Arro
Taksit.fi - national catalog for finding local taxi companies in Finland
Taxibokning - a service for booking taxis within Sweden
Taksod.net - a portal about taxis in Estonia
monTransport.com - a taxi search engine comparing local taxi companies and fleets in France.
MyTako.com - a cab fleet comparator in France.

Car rental aggregators

AutoRentals
QuarentaCars - compare regional and local rent a cars agencies in Spain
Rentalcars

Intermodal / Multimodal aggregators

Nowadays, specialized aggregators compare more than one mode of transportation, often including bus, train, plane, and even ferry or boat. Depending on the structure and their goals, those usually offer a "door-to-door " service: finding the best price for a complete journey combining all kinds of transportations. That said, they can cue you as to which companies are among those plying a certain route.

Tictactrip - online european multimodal company; they provides a complete transportation service while respecting the environment.
Omio - train, bus, flight and ferry companies
Rome2Rio - online multimodal transport search engine for inter-city and intra-city trip planning
Trainline - trains in Europe; do compare with the railway websites themselves though

Hotel and hostel aggregators

Large hotels and hotel chains have a variety of rates: prepaid, flexible, corporate, discounted, etc., and they adjust their rates based on demand. Some hotels sell rooms to these booking sites at a discount. The booking companies then price in their costs to determine the price they will offer you.

Some hotel chains promise you the lowest price if you book through their website. For various reasons, many hotels consider aggregators a necessary evil at best and while you might indeed get the lowest price when booking through a certain aggregator, some hotels treat customers who booked directly through their own booking channels better, especially in terms of discretionary services not explicitly part of your contract with the hotel. Hotel owners and especially small guesthouse owners are often happy if you tell them you found them online, but came in without booking, although many hotels have higher rates for those who didn't book. Another option is to tell the hotel directly that you see a given price on a consolidator and ask them to match the price.

Hotel aggregators include:

Hotels.com - offers one free night for every 10 nights booked
Hostels.com - hostels and budget hotels
Booking.com - wide selection of hotels available - you can almost always get 10 percent moneyback using link from WizzAir (you can find it when you login to WizzAir website)
Agoda.com - specializes in Asia
Priceline.com - allows you to set a price that hotels can try to match
Hostelworld
eflyto.com - has a wide selection of hotels worldwide

The air travel aggregators (listed above) also do search and book hotel rooms and car rentals.