You can use Audacity for Free but its not a "real" beat making software since it is limited to cropping and chopping music only. A good beat making software I can suggest is Dub Turbo however its not a free software. The only good thing is that its cheap software (at around $39).
You can use it to make your own beats, remixes, techno tunes, dubstep and other kinds of electronic music. Its worth the small investment.