There is what was Tonehammer - I think now some the choirs are under the brand SoundIron, and some under 8dio. There's Requiem, which is epic chants, legato sustains, and classic soloists. Soundiron only have the Kontakt hosted light version. They also have the somewhat creepy Mars male chanting choir.
8dio have Requiem pro containing masses more patches. They also have the Liberis angelic choir.
Vienna Symphonic Libraries have a few choirs which can come in bundle - Soprano, Alto, Tenor, and Bass. The sustained 'ah' and 'ooh' legato is very nice, and the software host id very nice and easy to use. Very easy to switch between the many articulations. The stacatto chants are not the best, but the other articulations more than make up for that.
Then there's Cinesamples' Voxos choir. It comes with a phrasebuilder like Requiem, but is a bit pricier I think.
I currently use Vienna Soprano Choir, Requiem Light, and EWQL Colossus' Male Choir for some of the deeper male atmospherics and shouts. Here's something I did using exclusively the Requiem Lite presets: http://soundcloud.com/jamasia/o-fortuna