Off the top of my head, Magix is free and Buzzware is free. Buzz is used by The Fats Sabbobah (if you are unfamiliar with his music, I recommend you change that). Basshunter uses Magix and I used it for my song "Killer Space Tomatoes". after a little getting used to, its somewhat flexible and easy to use. I can't say much about Buzz since my OS isn't compatible.
You can also use Audacity to manipulate and record sound files. You can essentially make your own instruments or even beatbox samples by using their sound generator and running it through some FXs. I made a few and used about three in "Judgmental Fish"
Here's a link to some free downloads:
http://download.cnet.com/Magix-Music-Mak er/3000-2170_4-10698847.html