From random search:

We ran into this website - Functravel.com and let us tell you why we want to post this website. It's because... it'll search the cheapest airfare all in one shot. Well-known website, Kayak.com does the same thing, but what we liked about this website is the straight-forwardness of the website. You type in where you are flying from and to, put dates, and hit go. All the filters that you can apply is easy to navigate also, and the results are listed in easy-to-read grid.

Check out: http://www.functravel.com/Flight/

And this is bonus for UK/GB Readers on Thailand Travel:

http://www.prweb.com/releases/2009/09/prweb2869274.htm