For a more eclectic and perhaps affordable set of options, use the site a friend recommended, from which I found my trip to Morocco: Adventurecenter. They aggregate trips from many companies, and have reasonable search options including price. Airfare is usually not included, and you'll find that it's the biggest issue with most of the more interesting destinations (read: off-the-beaten track). If you're afraid of traveling to "exciting" places alone (as a woman, I am), this type of group travel can be really fun.
For other resources on cheap travel, I recommend this site: Worldscheapestdestinations. It's not pretty, but the info is good.