Soundtoys MicroShift - The MicroShift emulates the sound of the famous Eventide Harmonizer, and anyone who's used one of the various hardware versions knows that there's nothing else that sounds like it. The same goes for the Micropitch (except for the Eventide's own Ultra-Channel). If you want to widen out a guitar track, make a keyboard more interesting, or smooth out a pitchy vocal, this is the plug to do it.
Lexicon PCM Reverb Bundle - Once again, there are a lot of great sounding reverbs on the market, but there's something about the Lexicon bundle (which includes a hall, plate, chamber, and room) that just seems to meld a track together so well. Just like the famed Lex hardware units that were the standard in studios for decades, Lexicon reverb plugins are hard to beat. (Honorable mention to the Pro Tools native D-Verb, which always seems to work when you need some extra ambience without having to sacrifice computer processing).
Universal Audio EMT 250 - I'm a big Universal Audio proponent and one of the best things they've done is a great emulation of the world's first digital reverb - the EMT 250. The 250's limitations were actually its strong points, with a limited bandwidth that means it always sits well in a track. Hard to beat for drum ambience.
Universal Audio MXR Flanger/Doubler - For modulation, it's hard to find a more versatile plugin. It does deep flanging and excellent doubling (although the Boss CE-1 admittedly has a sweeter chorus), which works really great when it comes to subtly widening a keyboard or just plain making it more interesting. I find new uses for it all the time.
There you have it. You'll see at least a couple of these on every mix that I do.
If you want to learn some really great mixing tricks using some of the above plugins, check out my 101 Mixing Tricks coaching program.