Back in the day before Ableton Live and Reason and all the other sequencer apps out there, desktop electronic producers made do with trackers: apps which allowed the budding Moby or Paul Oakenfold to sequence samples. They were basically software equivalents of legendary hardware sample sequencers like the Akai MPC. These usually had all of the usability of a 1957 Trabant and none of the good looks.
Renoise 1.9.1 sequences like an old-school tracker, but it's got loads more features: plugin and MIDI instruments, effects chains, a halfway decent mixer, and even internal sample editing. Everything a growing music geek needs to make bleep-bleep music (and maybe more). It's available for Windows, Linux and Mac OS 10.3.9+ as a Universal Binary.
At 49.99 euros (US$75.80 at today's exchange rate) for a license, Renoise is a helluva lot cheaper than alternatives like Live or Reason, and the ability to use a single license for versions running on multiple platforms is nice. The only drawback is that the interface appears a little complex for users unfamiliar with the conventions of sample trackers. Also, the demo version times out and doesn't allow rendering of your tracks out to .wav format.
I still have nightmares about using FastTracker on my old Pentium II back in the late '90s, so I haven't tried this one myself. If you have, drop me a line in the comments and let me know what you think.