First you should listen to the sample above to give you an idea of what mastering can do for a mix. This is of course different for every genre, but it will give you some idea of what a good master can accomplish from a good mix. Remember a poor mix can not be fixed in mastering. Mastering may help your mix, but if the song is not mixed properly then it will never achieve the results that you are looking for.
Free Bomb Factory Plugins, Including the Mastererizer.
Voxengo Mastering Plugins, have been around for a long time.
NWEQ, Parametric EQ.
BuzMaxi, Mastering Limiter.
AAMS Auto Audio Mastering System
Ozone by Izotope has become one of the all in one mastering plugins that many mixing engineers and artists will use when trying to master their own music.
My recomendation is to try Waves VST Mastering. With Waves you can download and demo their Masters Bundle or Grand Masters Bundle. This VST Mastering Software can then be used in your audio program of choice as a VST plugin.
If you want some more information about mastering techniques please check out our Audio Mastering Tutorial, The Mastering System. This guide will help you to understand what mastering does and how you can apply these techniques to your own music. We are dedicated to helping independant artists as much as we can. Providing the links above and tutorials on our site is one way we can help.