1. Wikitravel
Wikitravel is a crowd-sourced guide for every conceivable destination. I especially like the sections that detail how to get around, as the details are recent, and regional scams are outlined.

2. TripAdvisor
I use TripAdvisor as a rankings system for sightseeing attractions, hotels, and restaurants.

3. Matrix Airfare Search
One cannot actually reserve flights through this Google property, but the engine is fast and allows for routing codes, which has its own syntax. Like most of the major search engines, low-cost carriers are not included in the results.

4. Google Maps
In addition to using the basic Maps function, I like to plot my travel with the Maps Engine, which you can access by clicking “My Places” –> “Create Map”. Then, I can color-code and drop pins for a given city and measure distances quickly. Creating my own travel maps provides me with the spatial information for planning a day or booking a hotel, where location is paramount!

5. Openflights.org
I discovered this resource recently, but it is invaluable for the frequent flyer. You can enter a given airline or airport, and OpenFlights.org will visually show you all of its routes! If you are a programmer, then you can also use its open-source database for your own purposes.