Figured I'd start this thread to get some stuff out there that people might not be aware of. I'm coming from a Windows-centric perspective, so I'm only listing stuff I know works nicely in Windows (sorry if you're running Vista, I can't confirm everything. :P)
Digital Audio Workstation software Kristal Audio Engine
Probably the easiest free DAW software to get in and out of. Full VST plug support (though it has no midi so no VSTis). Fast. Comes with a pretty nice set of plugins that you can use in other hosts if you like (I like the compressor). Definitely worth an install.
Traverso DAW
A free, open-source, cross platform (runs on Linux, Mac, and Windows) multitrack recorder with a keyboard-heavy workflow.
Music Studio Producer
Hell if I know, haven't tried it yet.
Temper
A robust audio/midi multitrack software with a heavy emphasis on midi. Designed for creative manipulation of midi parameters and stuff. There's a beta VST version on the forum. Not freeware, but unexpiring uncrippled trialware--basically, when you start using it seriously, dude wants his 50 bucks. Oh, and there's no installation, so it's totally portable.
Multitrack Studio Lite
VERY compact, VERY powerful multitrack software with strong MIDI support and a unique interface. Free version is track limited.
TrakAX
Self-touted as "the Ultimate creation tool," it basically is a universal timeline software. Insert pictures, video, and audio. Also supports DX plugins and has strong beatmatching and time/speed alteration capabilities. If nothing else, grab it because it's better than Windows Movie Maker.
Audio editor Wavosaur
A shockingly powerful, totally free, absolutely TINY audio editing application. Supports VSTs and, aside for multitracking, seems to leave Audacity in the dust with regards to features and power (and Audacity is a nice application).
Fun toys I haven't really gotten down yet Linuxsampler (which, despite the name, runs and runs pretty well in windows) Gigasampler compatible sampler program that runs crossplatform. Website has links to a number of good GIG files, including a 992MB grand piano. It will eventually run as a VSTi, according to the development roadmap.
SynthFont is a player/synth that loads GIG and soundfont libraries for MIDI playback. Also runs as a VSTi.
Fluidsynth. For description, read Synthfont description.
MuseScore is A WYSIWYG notation editor. Sadly, the Windows version doesn't support midi yet.
Other TrakAx TrakPacks
Remember that TrakAx program I told y'all about up ^^ there? one of the most intriguing things about it (and what they're apparently marketing with the free software) is the royalty free sample packs they carry. There are several free ones, and the ones that cost run 2 to 4 bucks apiece. There's some pretty darn nice stuff in there, too.
That's all for now. Feel free to add on. This is the most I can do right now since my music pc is dead.
Free Software (DAW, VSTi, FX, etc) 
