Hi, I have used several different discount ticket sites and truly love (and highly recommend) www.Hotwire.com.
The site is easy to navigate and understand. As long as you are willing to be flexible on which airlines you are flying and the exact times, this is a great place to find wonderful deals. You can use their search engine to see what prices they are offering without having to make any commitments (unlike Priceline). Registration is free, and their customer service is very kind and helpful. I've purchased tickets several times using Hotwire and always had a positive experience. And their deals are fabulous!
One trip, our flight was cancelled at the last minute due to mechanical problems and not only did they contact us before hand to let us know what was going on (a kindness which I had never before experienced) but they arranged an alternate flight for us and helped us get "bumped up" to first class. I was just thrilled!!
One hint, you may find much different pricing depending on what days you are flying. It is usually best to do several searches, using a few different days for your trip and see how the rates change. Sometimes flying on a Tuesday (as opposed to Monday, for example) can actually save hundreds of $$.
Hope this helps and happy travelling!