ITASoftware - The ULTIMATE free search tool for cheap plane tickets. You don't actually book through this site, you do your searches and then book either through a discount site like Orbitz or directly through the airline's website. This site will search many airlines at once, shows you all the options sorted by price, date, etc... BUT the best feature is the weekend search/month long search. If your travel dates are flexible you can very easily see which days are the cheapest narrowed by airline, number of nights stayed, etc. I know some other travel booking sites do this, but this site has no graphics so it loads faster and is arranged in a much more user friendly way. It's commonly used by travel agents, but you can login as a guest and get unlimited free access to a very powerful search tool.

Orbitz - because I can see seat availabilty without having to go through the purchase process. When picking a flight I want to be sure I can get the perfect seat for me.

Southwest Airlines - because they don't show up on the ITA search and do have some cheap prices.

JetBlue doesn't fly out of my airport, but I would use their site, too, if they did. I think they're a cool company.

Hopefully these resources will save you a few bucks in the process of booking your next trip.

Tip: When booking close to the departure date (vs 3 months ahead of time) don't forget to consider 2 one way tickets instead of a regular round trip. I recently saved over $100 by booking 2 one ways using different airlines, and had a lot more flexibility in choosing the best flight times for me.