All of them, not to bre flippent, but seriouslly try everything you can get your hands on, free or demo, see what you like, but DONT keep everything.
I advise on 3 folders, 1new, 2like, 3 keep.
If it's still in the new folder after a month without use, lose it.
If it's still in the like folder after 6 months, decide if you really need it, or if something else does it better faster, differently.
And set budgets for purchasing the plugs you really like, even if you have the dough, allowing yourself time to learn thouroughly what you have is vital towards getting your money's worth. But to answer more directly, for mac.
Crystal-great synth, well worth more than the price of nothing.
all of the dfx plugins.
any SmartElectronix plugins, especially these for real fuckery.
there is quite a bit of wonderful work by amazing people out there. annnnd,
when you find one, don't be afraid to let the rest of us know.
oh and if you like glitch
http://www.defectiverecords.com/majormalfunction/
for mac.
(my next "budgeted" small develper purchase
So I thought I'd ask if there are any free VSTs that you guys think are must haves.