In reference to the plugs question:
Being a bit of a "plugin in 5.1" guy, there's quite a few legit 5.1 plugs now.
Of course there's no need to talk of EQs, because being static devices you can link them as multi-mono into whatever channel config you like.
Being a mastering forum, there's not much need to talk 5.1 reverbs but Digi's own ReVibe is true 5.1 and Sony are promising their Oxford Reverb will be 5.1 at some future time. Waves IR360 is a true surround reverb too which drinks your CPU heavily but G5s can handle it, but they also have a reasonable reverb in their TDM 360 bundle.
Dynamics wise, there's Digi's Impact and Smack! compressors which offer true 5.1 modes. The Waves 360 bundle has a fixed ratio and knee [ie very soft] 5.1 compressor with lots of different detection modes for its three detectors. Their L360 limiter works the same, supposedly providing L2 style limiting in a three detector 5.1 environment but without any release controls. This is the only true 5.1 limiter plugin within the Pro Tools environment to my knowledge.
I'm sure there's more 5.1 stuff but not being in front of the system at the moment I can't check, but the Waves 360 bundle alone can really get you going. It's not cheap and it's not separable but it covers all the bases reasonably well in one hit, including bass management and folddowns - plus there's nothing else competing with it, making it pretty much compulsory unless you want to do all that stuff outboard, which is certainly possible but even more expensive (read TC6000 etc).
