I think everyone has a favourite mastering weapon, that last minute icing for the cake. For me, I'd say it's Venn's V-Clip. I don't use it for all projects, but when it comes to taming transients and smoothing out levels transparently, V-Clip is it. Its visualization alone is worth the price of admission. The fact that there's also oversampling makes it a winner, especially if you use V-Clip for its distortion and saturation abilities. Definitely a keeper.
In a way this is the next version of Free Clip, but with more features, yet the same minimal CPU footprint and the same fast loading time. The sound is great, it has cool oversampling features and there are even distortion and a wavefolding distortion option. Furthermore the is the custom section, where you can combine such algorithms for the posiotive and negative site of the audio signal and do even an intended offset.
What made me buy this little cool tool is the GUI. It is so nice to use and the visual feedbacks of the signal is just great. More plugins should have such graphics. Totally worth the money; while it even does not cost that much at all! Thanks to the dev! (=.