Levelator requires Java 1.5 so it needs at least Mac OS X 10.4, but it's Universal Binary and free for personal non-commercial use. Runs on Windows too, if you care. It's in very early beta (Version 0.1b). The download page says "...the download and install mechanism is a little sloppy. We'll be making this much nicer over the days and weeks to come, but we didn't want to hold back The Levelator, the core functionality of which is far more solid." In my limited testing this evening it seems to get the job done. I used a 20-second clip of a voicemail message that has 2 of us speaking (one of us actually yelling from across the room) and the original levels are horrible, as you might imagine. A split second after I dropped it onto The Levelator it was done and the difference in the playback audio was impressive. I also found a pointer to How-To use The Levelator with GarageBand's compressed files - which I think I'll be sending to our resident Podcast producer (I'm talking to you, Chartier) here at TUAW HQ.
If you dabble in podcasts yourself, download this puppy and take it for a spin. You might like it!
[via Steve Rubel's del.icio.us links]