I don't remember if I heard of her first in my music lit classes, or when talking about different kinds of music with my composition friends. Her and Ockeghem are by far my favorite composers of western chant. Their music is incredibly haunting. And if you like her, you should really really check out Richard Einhorn's Voices of Light! I'll post a video of it later, though you can't find any of the full pieces on youtube. Anonymous 4 performs in it, and they've done a lot of stuff with Hildegard's music as well.
Speaking of chant: I'd heard a while ago on some forum (don't remember where, but don't think it was here) that groups were starting to record what has been found of early Irish chant. Masses for Saint Bridget's Day and such. Anyone know where I can find these, or know what I'm talking about? It's been driving me crazy because I'm sure I bookmarked everything,