Hotwire is a vacation booking service that provides deep discounts on highly-rated hotels. They also book flights and car rentals.
Expedia is an online travel agency that allows travelers to discover and book flights, hotels, car rentals, and more.
TripAdvisor is a travel, restaurant, and flight booking online service. They also operate active forums and leverage user-generated content to show their services.
Kayak is a travel agency and search engine founded in 2004 that helps travelers find and book hotels, flights, and rental cars.