To get a free start (the only free one I know of) Acid Express.
Limited to 10 tracks but you can create a composition using pre-recorded loops.

http://www.acidplanet.com/downloads/xpre…

A free loop pack can be downloaded from the site.
You can get more Acid compatible loops from Macloops:

http://www.macloops.com/

And more from free-loops.com

http://free-loops.com/

For commercial apps:
Mixcraft:
http://www.acoustica.com/mixcraft/
Sony Acid pro 7:
http://www.sonycreativesoftware.com/acid…
Dance EJ:
http://www.techspot.com/downloads/3146-d…
FL Studio:
http://flstudio.image-line.com/

All have free trials.