Not quite free, unless you get a new machine. check out Gararge Band, part of the iLife suite $49 upgrade for all 5 apps.

It's sort of a cross between Reason and Acid.

It has software instruments, that are simplified versions of the Logic synths.

Also it has buit-in effects, and supports Audio Unit instruments, and effects.

The Apple Loop technology was developed by the software team that created Acid. They are now Apple employees.

Apple loops have an audio, and a midi component.
You can edit an Apple Loop's midi, change sounds, rename and save. and it will create the audio component on save.

Comes with over a thousand audio and Apple Loops.

Over a dozen Garage Band sites have sprung up where people are sharing their creations.

8)

From what I have heard, PT Free will be coming out for OSX in a few months. I heard this from my dealer so I don't really know exactly how accurate the info is. Digi has been really busy with a lot of other projects like ICON, Command 8 and the recent Bomb Factory purchase so maybe they will be able to get back to work on it now that most of that stuff is all done.

Audacity.

http://audacity.sourceforge.net/