Players support

Replaygain being present in the specs of flac, mpc and ape formats, any player that support those formats usually support replaygain.

The situation with MP3 is rather different, as it was not part of the mp3 specs. The APEv2 tags metadata implementation is somewhat becoming the de-facto standard.

Windows

Foobar2000 supports replaygain in all possible aspects. (it can also calculate replaygain)
Winamp. MP3 replaygain support with adequate plugins (e.g. Otachan's in_mpg123 - if your japanese is not that good, you can find details in this thread)
XMPlay recently implemented replaygain

and probably others.

Linux

XMMS. Reads replaygain from FLAC, MPC, ogg ..

For mp3, use the CVS version of the xmms-mad mp3 plugin (it's not yet released as binaries, furthermore not available in distribs' versions for now. Meanwhile binaries are available there : custom binaries)

amaroK. By using the amaroK-script ReplayGain

and possibly others, since TagLib added support for APEv2 tags in mp3 files, players using this library (like amaroK and JuK) might support that kind of replaygain tags in the near future.

Portable devices

Current development builds of Rockbox for the iRiver H100 series players support replaygain for some encoder formats. This is a rapidly evolving feature. There are no other portable players known to support replaygain.

The iPod features Soundcheck, which seems to produce roughly the same normalization gains as replaygain, but doesnt provide an Album Gain.