MP3Gain is a program that analyzes MP3 files to determine how loud they sound to the human ear. It can then adjust the MP3 files so that they all have the same loudness without any quality loss. This way, you don't have to keep reaching for the volume dial on your MP3 player every time it switches to a new song.
MP3Gain is an implementation of Replaygain, supporting Track mode and Album mode. However, with most other formats, the necessary loudness adjustment of Replaygain is stored as metadata, thus leaving the encoded results alone. With MP3Gain, the loudness adjustment is done on the data itself, albeit in a lossless/reversible way. Another difference with MP3Gain is the fact that it can only adjust physical volume in 1.5 dB steps.