I'm consistently humbled by the philanthropic souls developing free audio software. The provision of free tools for emerging producers is playing an important in the democratisation of audio culture. If I were to step into my time travel box armed with my accrued technical knowledge, I'd ditch my corporate tools and join the revolution.
My pick of free audio tools for Mac OSX (some are cross-platform, so it's worth checking):
It's fruity (watch your speakers!), and mainly good for creating glitchy weirdness, but Guido Smider's small-but-complete audio environment gives you a primitive drum synth, drone-bass generator, assorted FX and 4 malleable audio tracks.
Jeremy Wentworth's app is a JAVA-based granular sampler capable of creating unlistenable noise from the most polished studio productions. Touchscreen control must be ace if you're yuppie enough to own an iPhone/iPad.
A nice, graphic-pattern based generative music player designed by Lawrie Cape. It has its own plinky-plonky flash sounds, but can be connected to a soft synth via Flash Midi Server. A great song starter - you'll be creating face-melting, off-kilter melodies in no time.
The ever-industrious Patrick Kunz makes a vast array of VST and AU plugins (FX and synths), which can be a little buggy, but sound excellent.
A comprehensive VST/AU plug in pack, with including some sweet amp/speaker simulations.
I have no idea what spectral processing is, but it sounds pretty cool! Michael Norris makes this massive AU plugin pack.
Ever needed to calculate how many samples are in a quaver at your current sample rate? Need to calculate how many milliseconds are in a crotchet at your current BPM in order to sync a hardware delay unit to a beat? Music math is a humble but useful tool, especially for inferior mathematicians like me.
Amazing for trouble shooting MIDI problems in a system, this allows you to spy on all the MIDI information buzzing around.
There is of course Audacity audio editor, and the DAW Ardour, which I've never used, but am giving an honourable mention.