First off, what SW program are you using? Pro Tools, Sonar, etc.
Second, in all of the years I have been doing this I have not found any type of mastering SW that was of any quality for free, especially a brick wall limiter (those algorithms are not cheap and require some serious DSP power). The TC6000 has a great selection of mastering SW available including the best BWL I have heard. I recently used it on a project and was quite pleased with the results.
For PT, Waves makes a decent selection of mastering tools but again, I have never found any that are free.