Great for: budget accommodations
Booking.com is our go-to resource for most travel, largely because it's such an expansive listing of reasonably priced locations. You won't find too many large resorts or five-star properties here, but the trade-off is that it's full of good budget hotels, guest houses, and hostels. The pricing on Booking.com is competitive and worth a look; they have properties available in most large to mid-sized cities around the world. Bottom line: a very reliable choice for lodging and typically the first site we visit to see what's available.
Great for: living like a local
We've done nearly a dozen Airbnb stays now, and each one has been different from the rest. We've booked entire apartments - some of them really spectacular - and at times stayed budget conscious by sharing a space with strangers. While most Airbnb rentals lack the traditional trappings of a large hotel (including a concierge and other amenities), the upside is that you'll feel more like a resident of the city and will likely have access to at least a few of the comforts of home, like a kitchen. The biggest plus for us is that many Airbnbs (depending on the city) will have washer/dryer units in them, which is a huge benefit on a long trip when you need to wash your clothes. It's also nice to stay in housing with its own character rather than a hotel, which, despite its positive aspects, can feel like at times like a cardboard cutout.
Great for: guest houses (especially in Asia)
Agoda has one of the most comprehensive listings of guest houses, hostels, and budget hotels specific to Asia. For those visiting the region (especially frugal travelers), Agoda should be your first stop for lodging options. Their rates are hard to beat relative to competitors.
Great for: tons of hotels
Expedia is a juggernaut in the travel industry, providing a platform for travelers to book hotels, flights, cars, cruises, excursions, and more. For hotels, you'll find a vast list that includes all of the major chains and resorts (Marriott, Starwood, Hilton, and others) as well as some independent locations.