Voxengo makes some amazing plug-ins. Elephant has been mentioned, but I'll throw in Crunchessor as one of my favorite track and mix compressors. It behaves well, and covers a LOT of sonic territory. I use it frequently when I'm not in the mood to use an emulation, and being a dinosaur I do tend to lean towards emulations of gear I 'grew up' with, but that's me.
That said, I think Izotope makes perhaps the most easy to use, musically useful equalizers and dynamics processors! Their presets are usually great jumping off points.
I don't own Ozone (yet) because I have tools that do the same things, but maybe make me work a little harder - which is ok, since I've already invested in them. That can make a difference<G>!
That said, and I get flamed for this but I'll say it anyway - mastering is NOT about the gear, although there is gear (hardware and software both) that is optimized for mastering. Mastering is a process, and more to the point, it's about the ears of the mastering engineer. The number one reason to send your tracks to a mastering engineer is their ears, and a close second is that they represent a second set of ears - ears that have no baggage with respect to the tracks.
I think that an awful lot of people are using the term mastering in ways that it was not intended, or rather in a very different context than the one with which I grew up.