My favorite free ones are from Valhalla, TAL, and Noise Engineering. Melda has a ton of free utility plugins that are useful for all kinds of things - not flashy but lots of useful effects and some utilities like an oscilloscope and audio analyzer.

Also, sign up for mailing lists for the plugins you want but cost too much $$. If you can wait a lot of them offer steep discount sales around holidays and various other times. Two other plugins that I use a ton, decapitator and TAL DubX, I got at like 90% off.