Gentoo users: I just packaged rsgain! It's a command-line ReplayGain tagger1 with colorful progress text designed to be super easy to use.
I was impressed with it, I gave it a list of folders I wanted it to touch and it built a list of compatible music files and scanned them all. It's an easy recommendation if you've got a music library to manage and don't mind using a terminal.
If you manage your music with MusicBrainz Picard there's a "ReplayGain 2.0" plugin that will use rsgain as a backend.
You can get rsgain on your Gentoo box from my overlay:
eselect repository add sunset-repo
emerge --sync sunset-repo
emerge media-sound/rsgain
It's also available for other Linux distros and Windows (but it's still a command-line program on Windows).
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It analyzes music files and writes information about how loud they get for music players to use
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