Never really needed any.
Everything I ever needed to do was actually available/possible within Photoshop itself - either for retouching, or for whatever other type of work (digital painting, compositing, masking, color/tone grading, sharpening, advanced postproduction, design ..) ..
But - some people like working with plug-ins, though.
For various good reasons (there are a few plug-ins that might be worth trying out, as far as I know, for some type of workflow ..) ..
Perhaps someone else will give you some better answer/insight.
if you ´jumped´ right from CS2 to CS6 - this is pretty a ´big jump´, since some major/significant improvements and functionality were added since then ... so now - I guess - for sure you´ll find a lot of new useful things and functionality to play with, within Photoshop itself