There is just so, so much, and I can only really mention stuff in the Christian tradition. Think hundreds of years of religious music. You can go whereever you want with it - from Shaker songs, to Palestrina, to Sacred Heart Music, Eastern Orthodox (check out Fairuz, a Lebonese Maronite Christian who sings in Arabic), to modern choral music (check out my recent post in the "What Are You Listening To" thread - the record is called Scattered Rhymes by Tarik O'Regan, De Machaut and Gavin Bryars. It is wonderful choral music (Estonian Philharmonic Chamber Choir). If you are looking for more modern acapella stuff, please check out Stockhausen's Stimmel. It is beyond words.
Anyway, for a start, check out some American sacred heart music. Read up about it on Wikipedia as well.
http://www.youtube.com/watch?v=eoPWLov546M
I've also been listening to the Boston Camerata a lot recently as well.
http://www.youtube.com/watch?v=kVvJC36L4yI