Another freebie mutliband compressor is Triplecomp (actually has a nag screen but works well in free version inside PGMusic products). The author actually took the time to fix a couple issues that had previously existed when used in PT.
Yet another nice free one is Grancomp3 (I think that is correct name)
http://www.kvraudio.com/get/1047.htmlI also like Ozone4, and think it is well worth the money. I think its over $300 though, not the 200 mentioned above. It has great presets for the beginner to start from, and contains many FX all in one module; more than you could otherwise get in the four slots available in the RB final output.
It also contains a great dithering setup, which is a whole 'nother discussion..
RobH gave a nice simple explanation for getting started.
I set this site up to help people get their music ready for mastering, which is an important step-
http://musicmastering.orgThere are a few mastering tips in there.
I've had a couple other mastering engineers contribute articles and am still looking for more to pitch in.
Thanks for the link to reaplugs, jscpro40, I've heard good things about those.