AnalogX: Sayit, Vocoder, Rhyme;
Big Tick: Angelina, Hexaline, Dodecaline, Mabento, TickyClav;
ZynAddSubFX
AnalogX has all kinds of nifty stuff. Sayit is an old-skool low-fi text-to-speech converter with lots of character. Rhyme is a rhyming dictionary with TONS of entries. You know what a vocoder is.
Big Tick made Rhino, my favorite synth, but they also have some freebies and demos worth having. Angelina is a "breathy" synth. Downloaded a bunch of patches to go with it. Hexaline and Dodecaline are delay plug-ins. Mabento is a ring modulator. TickyClav is a free program that attempts to emulate the ultra-funky sound of the Hohner Clavinet."
ZynAddSubFX is an open-source synth I like for one patch: the Space Rhodes. I used it in the Open Source Jam. Unfortunately, they still don't have a working VSTi version of it, so it is standalone only. More unfortunately, I could not get MIDI-OX working under Vista, so I don't have a convenient way to feed MIDI to it. (And no, playing a keyboard real time is not convenient!)