iZotope Spectron... God you can do bizaar stuff with this plugin. Really neat idea, and I haven't seen anything like it except for that new plugin "EQUO" (which is useful for other things too) in the latest version of FL Studio. If it had the ability to route output from the various split frequency bands to internally hosted VSTs and automate them via LFOs/ evenlopes/ external MIDI it'd be just about perfect.

iZotope Ozone... Yes I am a fan of this company

Waves MaxxBass. For THICK basslines

Sendarella: Extermely useful freeware VST plugin useful for routing sound between programs and routing paths that aren't supported by your VST host.

KORG MS20. Nearly as cool as a MS2000B

EDIT: How could I forget - REAKTOR!!!