Windows only: MP3 Quality Modifier makes it easy to lower the bit rate of your audio files, allowing non-audiophiles to more of them onto an MP3 player.
Lossless formats and high bit rate MP3 encoding is great when you have tons of space to spare, but if you have a lower-capacity MP3 player, you might want to trade true fidelity for a wider variety. MP3 Quality Encoder facilitates that trade-off by adjusting the bit rate of MP3 files in bulk, reducing their sizes. While dropping an album from 320Kb/s to 160Kb/s might be an extremely noticeable shift when the song is being cranked out of your high-end home stereo, but it's likely the difference will be fairly negligible when you're rumbling along on your subway commute.
MP3 Quality Encoder starts out by asking for a target bit rate mode and rate, along with stereo settings and sample frequency. There are a handful of presets like "high quality" and "portable" for fast, uncomplicated operations.
The application is under 400k and portable, which makes it easy enough to load onto an MP3 player itself and take it with you. MP3 Quality Modifier is freeware, Windows only.