First, on dubstepforum.com and idmforums.com you'll find epic lists of good VSTs. Also, a good place to find info on VSTs is kvraudio.com, a site which keeps a MASSIVE database of virtually all plugins.
Schwa Olga is an excellent synth. It's more or less like a Minimoog in function but sounds extremely dirty/grungy and can be distorted in cool ways. It's cheap but not free.
Oatmeal is a very good-sounding two-oscillator synth. It's free and lightweight, but I don't really like its programming style (I'm a synthesize-by-numbers kinda guy, haha).
Nitrox is also free. It doesn't sound quite as nice as Oatmeal to me (more dry, clinical), but it's very flexible with custom waveforms and shapers and such.
Crystal is free and mind-blowingly good but it's very dense. There are pages and pages of settings. If you get comfortable with it you can cause some serious madness, but if you're not yet accomplished as a synthesist then rather stay away for now.
Togu Audio Line has some excellent effects including a Moog-style ladder filter, a bitcrusher/sample rate reducer (excellent for Justice-style filth), and a tube emulator.
JB Ferox is another good free plugin. It's a tape simulator which does wonders to bring a tightness and punch to mixes. I also often use it on orchestral stuff, as well as piano and distorted electric guitar.
...I can't think of any more now. Message me if you want more, then I'll go dig through my VST folder. ;)
Oh, wait, one more: SimulAnalog guitar suite is excellent for beefing up basses and leads.