The Levelator is offered by The Conversations Network as a single-purpose tool: it takes uncompressed audio (WAV or AIFF files) and performs a small miracle. The file is leveled to a uniform loudness level, even if varying parts are recorded with different microphones, audio settings, or even in diverse corners of the world (if you've ever tried to record a podcast over Skype, you know what I'm talking about). While many audio apps have normalization or 'leveling' functions, in my admittedly amateur audio experience I haven't heard anything like The Levelator; those with more savvy in this area tell me that the output is akin to what you would get with a human engineer 'riding the meters' to adjust the sound dynamically as it varies.
The really nice thing about The Levelator -- and this is an odd thing to say about a Mac application -- is that it has, for all practical purposes, no controls. Drop a file on it, wait an appropriate amount of time and watch the blinking lights, then take your output file and continue on your merry way; the final file will simply sound way better than the original did. It's made my life much easier in editing the TUAW Talkcast, and if you have any hand in producing spoken-word audio it might do the same for you.
The Levelator is a free 48 MB Universal Binary download, and will work on either 10.4 or 10.5, as well as Windows and Linux. Enjoy!