Solid State Logic (SSL) offers three cool plug ins, one Compressor from the famous SL4000 E Console.
X-ISM. I use it now as a MUST have meter. It watches for Inter Sample Peaks which go undetected by most every other meter.
Halloween sound EFX
Voxengo has a nice selection of handy free AU/VST plugins
There is a Tube Amp simulator, some reverbs and delays, a stereo widener, and spectrum analyzer.
Also check out some of the free synths from u-he: