RightClickMP3 is a small and handy little utility that converts most popular audio formats to mp3, directly from the right click menu in Windows Explorer or Mac OS Finder.
It's designed to be simple and lightweight - it doesn't install anything else on your computer, and won't slow it down. It can easily handle batch conversion of a folder of files (even folders containing varying file types), and there is a bitrate selector for you to adjust the quality setting.
It supports Mac OS X 10.5 and 10.6, and all flavours of Windows (32bit and 64bit). Supported formats are WAV, AIFF, M4A, OGG, MP2, WMA, FLAC and AAC.
RightClickMP3 is priced at $9.99, and is available now as a direct download.