I started using Kayak.com when I was in college and looking for cheap flights home during holidays, and the site has only become more and more useful since then!  Some of its best functionalities:

1) Easy-to-use filters:  They have the "weekend only" option on the front page, but once you do your search, you can tailor the results down to what you really want:  specific takeoff/landing times, no red-eye, certain airlines, no turboprops, layover times, etc. etc.

2) Email alerts:  Since I fly frequently to certain airports, I set up alerts to tell me when prices drop below a certain price ceiling....