For free stuff check out;
Density MkII, Compressive Pro and Blockfish for compressors.
Pushtec, SonEQ and BootEQ MkII for EQ.
These are also useful;
Ferric TDS (saturation), Gclip (clipping), Gband (HPF/LPF), Nastly DLA (delay), Ambience (reverb).
A quick search on KVR should turn most of these up. Other than that I mostly use Reaper's stock plugins, they're great for very precise stuff. That should be plenty to keep you busy until you reach the point where you feel like you can justify paying £100 for a plugin.