sndplayer is a great program for .sndh-playback on atari, if you own an atari and haven't got it already... get it! :)
jam for atari which plays some of the known tracker formats, and winjam for win32 which plays .snd (sndh).
sc68 is an atari st and amiga music player, but there are additional packages for xmms and a winamp plugin.
deliplayer is a multi format player for win32, and when i say multi format, i mean it... over 200 compatible formats.
atari.org
www.myatari.net
atari-workshop.co.uk
atari-source.com
www.sospubs.co.uk - sound on sound.
tims atari midi world
www.yamahasynth.org
www.synthzone.com - soundbanks and other stuff for lots of synthesizers.
www.elektron.se - patches and os upgrades for the sidstation.
www.dhs.nu - dead hackers society, a great portal for atari users.
www.chiptune.com
www.chiptunes.org
www.creamhq.de/ymrockerz - this is where we revolt against the record industry with written word and ym2149 noize!
www.scenemusic.net - streaming scene music radio online.