Libera, http://www.libera.org.uk, has an "Enya meets Gregorian chant" sound, or maybe it's "Gregorian Chant meets the Third Millennium." Fabulous, fabulous, fabulous. I forgive them all the misspelled Latin words in their lyrics posted online. (Maybe they've fixed that since upgrading to their own domain!)
Adiemus, http://eden.vmg.co.uk/adiemus/html/menu.html, the music of classically-trained Karl Jenkins. His style draws from various traditions, and uses voice and syllable as another instrument. I am wild about it.
Solesmes Monastery, www.solesmes.com, THE place for Gregorian chant (the original, I mean). Their re-founder pretty much started the renewal and rediscovery of authentic Gregorian chant, and over the century and half since then, they have done innumerable studies, prepared critical versions, and done recordings. They also have recordings of organ preludes and stuff like that. Site has audio samples.
Choral Public Domain Library, www.cpdl.org, a wonderful resource with links for sheet music and audio files (midi and mp3) for music in the public domain! Searchable by composer, title, genre.
Women in Music Foundation, www.rtmol.stt.it/donneinmusica, promotes music composed by women "in every era and of every genre from antiquity to the present day." Also publishes musicological studies on the theme. Supports and offers recognition to women composers around the world. Sponsored a great series of concerts in Italy throughout the Jubilee year.
Pueri Cantores, www.puericantores.it, Italy-based association of classical boys' choirs.
Chicago Symphony Orchestra, www.cso.org, world-renowned--and right down the street from us!
Paul Schwartz, www.paulschwarz.com, lovely ethereal sounds. He has two albums loosely based on chants (State of Grace and State of Grace 2), plus two albums I still haven't heard (Aria and Aria 2), but am looking forward to maybe sampling at Virgin Records down the street. Kind of like Karl Jenkins' music, but less edgy. I like it all!
Paul Soniat, http://www.paulsoniat.com, my cousin's husband. A New Orleans sound.
Michael Card, http://michaelcard.com, my favorite Christian composer. Great lyricist.