for choral music:
http://www.cpdl.org/wiki/index.php/Main_Page
for instrumental music:
http://imslp.org/wiki/Main_Page
they both look like wikipedia, & I think they're like wikipedia in that everything on there seems to be in the public domain. the first one has lots of scores & it looks like the 2nd one might be older because it doesn't seem to have so many. i for one sometimes try to follow along to a piece i really like. anyway have at it everybody!
Good of you to start this thread. :)
Copying urls over to here would be too tiresome so I'm not copying the entire post but here is a link to my post on this subject (http://www.good-music-guide.com/forum/index.php/topic,13808.msg410887.html#msg410887) on the old GMG.
Also, there's an IMSLP (http://www.good-music-guide.com/community/index.php/topic,405.0.html) thread (it's over here, on the new GMG, not the old site).