FareCompare- They have a great map feature called where to go. You simply put in where you want to leave from and it pulls up the entire world of airfares. You can narrow them by how much you want to spend as well. Simply click on the rate and it will give you the exact location, rate, cpm, and airline. I don't like this site to actually book or pull up exact flights, but for gettign the low rates and ideas, it is great!
ITA Matrix-This is a program ran by google and really easily to put in exact destinations and gives you a rate for the month as well.
The Flight Deal- Nate gave me this site and its really easy to use. This is a blog where you can check each day and see what kind of deals are around.