Here's a tool that lets you convert a wide range of file formats into MP3 format. AudioAlchemy MP3 Edition gives a wide swath of control over speed, sound quality, bit rate, frequency and VBR/CBR. It handles a broad range of formats for easy conversion: WAV, MP3, WMA, OGG, AIFF, AU, VOX, RAW, PCM, GSM, ADPCM, G726, G721, G723, MPC, MP, and more to MP3. Also lets you convert existing MP3 files. Conversions are handled at the source, so the operation is fast.
The interface design probably won't win awards, but the drag and drop operation and speedy conversion more than make up for it. Other options let you enable or disable various features, such as Joint Stereo, Bit Reservoir and Short Block.