"Replay Gain is the name of a technique invented to achieve the same playback volume of audio files. It specifies the reference level of 89dB and an algorithm to measure the perceived loudness of audio data. Replay Gain is different from peak normalization. In peak normalization, you merely ensure that the peak amplitude reaches a certain level. This does not ensure equal loudness." - Hydrogen Audio Wiki

If your audio collection is made up of MP3 files then we'd recommend MP3Gain (note: MP3Gain, as of 2005, can also deal with ACC files via the AACGain plug-in). The real benefit of MP3Gain is that because it losslessly modifies the data, any player, regardless of whether it supports replay gain, will play the file at the correct volume.

However, if you have many different formats then foobar2000 has an excellent Replay Gain Scanner. This achieves the same result as MP3Gain but in a different way. Rather than reversibly adjusting the dB level of the audio, the Replay Gain Scanner writes normalising data into the tags of the file. This is then read by the player which adjusts its playback level accordingly. If you don't want to tamper with the file itself, you can choose to have the Replay Gain values saved in the foobar2000 database; this will leave the files untouched.