Some VSTs I recommend at least checking out off the top of my head:
Lennar Digital Sylenth1 (A very good and popular synth)
Native Instruments Massive (already mentioned)
Native Instruments FM8
Native Instruments Kontakt
Native Instruments Guitar Rig
Native Instruments Reaktor
Native Instruments Razor
Native Instruments Absynth
Celemony Melodyne
Dada Life Sausage Fattener (be wary of plugins with only a few knobs that promise to ''magically'' give you the perfect sound cough soundgoodizer cough, but a little bit of this could give a nice ''crunch'' to your sound)
iZotope Ozone
iZotope Trash
iZotope Alloy
(Look on their website for what these do)
Fabfilter Bundle
Waves Mercury bundle
Image-Line Bundle (Basically some of FLS native plugins, but you can use them in any other DAW, if it supports VSTs). I just discovered how fucking cool Harmor is a week ago. Can't wait to learn to use it.
Camelcrusher
Camelphat
OhmForce Ohmicide
PSP Vintage Warmer
Sonic Charge Bitspeek
Sonic Charge Synplant
sonogram sg-1
u-he Zebra2 (great synth used by Hans Zimmer)
Xfer Records LFO Tool
or alternatively:
Cableguys - VolumeShaper (and other Cableguys stuff)
Omnisphere
Trillian
MeowSynth
Some of these are freeware, others are not.
You should find them on one of the most famous public torrent trackers or on
http://audioz.info/If this fails, you could always try Google. I almost find almost everything this way.