Some songs are louder than others, and it's annoying to have to constantly tweak the volume knob to keep things at a reasonable level. Enter "replay gain". You use a tool to go through your music and add tags marking how loud each song/album is, then you play them with something that understands the tags and adjusts the volume for you. Very convenient. Robert Downes (Bobulous) has a nice post with scripts for tagging FLAC files (the heart of his scripts is metaflac --add-replay-gain *.flac). Then just add

replaygain "album"

to /etc/mpd.conf and restart mpd. Voilà!

If you're not using mpd, modern versions of flac come with an undocumented --apply-replaygain-which-is-not-lossless option. It's annoying that they haven't gotten around to documenting this option (since 2004!), but the syntax itself is pretty simple (description from this post):

--apply-replaygain-which-is-not-lossless[=<specification>]

where <specification> has the format:

[<preamp>][a|t][l|L][n{0|1|2|3}]

<preamp>    Floating point dB to add to the existing gain.
a|t         Album (a) or track (t) gain.
l|L         Peak-limit (l) or 6dB hard limiter (L)
n{0|1|2|3}  Noise shaping from none (0) to strong (3) when
            dithering back to integer amplitudes.

The default is 0aLn1. If you look in the flac source, this documentation is in doc/html/documentation_tools_flac.html (where it is commented out).