Mel Still answers:

If you are 18-26, EF College break has some good ones. I just got of a trip with them, and it was a lot of fun. Their discount code is hesseln1188 too by the way if you want to use them, otherwise, I use kKayak to find the cheapest airfare and Farecast to find cheap dates. Expedia also sometimes offers some really great cheap deals, but thats just a manner of stumbling upon it at the right time. Goodluck!