I knew I shouldn't have looked at this post...now that danged "On Eagle's Wings" is running around polluting my head.
And some more:
The Complete Works of Tomas Luis de Victoria - online in PDF, MID, and some MP3s
The Choral Public Domain Library - the best value in Catholic liturgical music (search "Palestrina", "Byrd", "Tallis", "Morales", etc.)
Another twentysomething's take.