My MP3 collection is about as eclectic as they come. I've got songs ripped from CDs, songs downloaded from countless online stores, and songs collected from, oh, let's say points unknown. Consequently, the volume levels are inconsistent across the library: Some songs are too loud, some are too soft, and some are juuuuust right.
Fortunately, there's a simple remedy: MP3Gain, a free utility that equalizes MP3 volume levels. It does so by modifying the appropriate metadata of each file so that music software and portable players know what the volume should be. Fortunately, it makes no changes to the actual music contained within each MP3, so there's no loss of sound quality.
Note: MP3Gain is free, but the author appreciates donations to help support further development.