Originally Posted by asterix2k10
Synchro Arts VocALign PRO V4
Audio Ease Altiverb 7 XL
iZotope RX 2 Advanced
Massey L2007 Limiter
If you're asking about "essential" plugins...and are on a budget, the first thing you should invest in for mixing in addition to the stock protools plugins is NR and a good reverb, and possibly a loudness meter.
The PT stock eqs and dynamics are certainly workable for many circumstances.
Once the work starts coming in, then invest in higher quality eqs, dynamics, etc.
(Stuff like Equilibrium is awesome, but avid channel strip or eq7 will, and does the trick.)
Add a Cedar (if budget allows) or WNS (if you're tight on funds). Izotope RX is handy, but not really great for realtime broadband NR.
There's a ton of reverbs out there....personally I wouldn't go with altiverb. (I own it but barely use it) There's a lot better verbs, that are much more flexible and easier to deal with on the mix stage.
Speaker phone is handy, but great futz type of effects are also easily attainable with a combo of eq, compression and verb.
McDSP N575 is something I find essential, but if I didn't have it, I'd use the avid channel strip.
I could go on....
Now if you're looking at sound design plugins, that's a whole other conversation......