Voxengo Peakbuster
Wow! Voxengo knocked it out of the park again with Peakbuster. I've long recommended TEOTE, and this seems to be the perfect-pairing! It's different enough that you want both.
I can't explain what it does -- all I know is it makes my songs sound better. Not in a way that has artifacts, like some other 'betterizers'... Peakbuster "just works" without adding any fatiguing side effects. It adds the kind of magic that when you turn it off --- you turn it back on as quickly as possible and think, "What would I have done if I didn't have this?!"
You think you're done with your mix and then Peakbuster adds something better that you didn't even know you needed, but once you hear it you don't want to remove it.
I'm using Peakbuster with TEOTE, Marquis Compressor, Elephant Limiter, and Span Plus. I EQ on my submix busses, so that's a perfect Voxengo mixbus mastering chain for me.
I'm always happy when I get a plugin that I can use on every track, and all 5 of those fit that bill. Peakbuster is outstanding!