BladeEnc (also available here) is a great MP3 Encoder. Unfortunately, it's a command-line utility. RazorBlade is a free graphical front-end that makes BladeEnc a lot friendlier to use. You'll have easy access to all of BladeEnc's options, as well as batch encoding, drag-and-drop support, and the ability to shut down the computer after an encoding session. RazorBlade can also minimize to the system tray while processing, plus do an automatic online check for new versions. RazorBlade and BladeEnc are a great combination for turning .wav and .aif files into MP3s.
