There are a bunch of free VST pluggins available out there, but they vary widely in sound quality. I’ve recently come across a good sounding “brick wall limiter” pluggin called Barricade. You can download it from this site: http://www.jeroenbreebaart.com/.
Basically you use the fader on the right to set your “brick wall” level. -0.3 is usually a good level to keep your track from digitial distortion. If you want to really maximize your track’s level, you can then turn the gain knob up which will of course make the track louder, but also hit the limiter more often/harder. I’ve found that this limiter is fairly musical sounding and doesn’t add a whole lot of color to the sound until you really start hitting it hard. It’s great to add to a narration or voice over track allowing you to get a good consistent level.