Free and pretty good. The newest version no longer requires Java . Versions for Windows, Linux, OSX. Much of the "pumping" audio artifacts in the older versions have been fixed.
This app is really nice for podcasts, speeches, audience recordings, etc. It's NOT meant for music, since it completely sucks the dynamic range out of the mix. Accepts WAV and AIFF files.

http://www.gigavox.com/levelator