Thanks to Jennifer for this tip! When we book our trip to Mexico in May, we're going to use Kayak. It's a travel search engine in the same vein as Travelocity or Orbitz in that it searches for the best deal on airfare, hotels, and rental cars; but it links you directly to the cheapest option (sometimes the airline itself, sometimes an agency), saving you the $5.00 or so that other search engines take as their share.
For instance a flight from D.C. to Cancun is $303 on Kayak and $310 on Orbitz. To get the cheaper fare Kayak links directly to United. That saving is worth a margarita on the beach, no?
For more information on Kayak, check out their site.