I'm looking for a free piece of software (or cheap) that I can put my music into, and it will mix them up, or something that allows me to do that. 10 points for the best answer!
(The program must be compatible on an iMac)

Virtual DJ

from the name itself, it allows you to SCRATCH, MIX and a lot of things. it looks like a REAL mixer.
what's good about this is that it's easy to use. no more reading of hard-to-follow manuals. you can learn this simply by trying it.