I know that you didn't ask for the following answer - but I have to emphasize that "mastering" is mostly about a set of different (and experienced) ears, not so much about gear and plugins. In an ideal world, you would ask a trusted colleague or - even better - a mastering engineer to give your work the final touches.
That said, there are quite a few plugins that can do the job, depending on the scale you measure them. iZotpe's "Ozone", for example. Voxengo's stuff, like Rob pointed out already. Waves, of course. Or the mastering plugins from Elemental Audio - namely "Neodynium". Sometimes it helps to add just a little bit of saturation and soft compression with PSP's VintageWarmer.
In the end, the treatment your mix asks for can be very diverse ...