Share any invaluable, awesome free plugins you know here.

I'll start us off:

Xfer OTT - Crazy multiband compression thing? If I'm not mistaken modeled after a preset in the stock Ableton Multiband.
Voxengo SPAN - Free Spectrum Analyzer. This thing is arguably my favourite plugin ever! Fun-tip: Hold Ctrl while clicking on frequencies to isolate them.
Voxengo MSED - Simple Mid/Side EQ I've taken a serious liking to recently.