Riverfolk. Their only album (so far), Meander, is absofuckinglutely fantastic. Anyone with any taste for acoustic music needs to hear them. They are fannish and like filk, but this is not a filk album (that is, there's nothing that I wouldn't expect a mundane audience to understand as readily as a fannish audience).
Three Weird Sisters. Best band in filk, period. They have two albums out, Rite the First Time and Hair of the Frog, and they are probably the two best recorded and produced albums in filk ever, the songs are mostly top notch (there are a few, but only a few, that don't really work for me), and they all have real talent. A lot of pagan themed stuff, and some other stuff that mundanes might find dangerously weird, but nothing that an open minded person shouldn't be able to listen to.
Leahy. My current absolute favorite Celtic band to hear live. Their first album (just called Leahy) is two steps down from hearing them live but that still leaves it one of my favorite albums. They have a couple more albums out that are good, but not as good as the first one.
Lunasa. Another awesome Irish band. They play traditional Irish jigs/reels/etc with a bit of a jazz sensibility, with string bass; it's just delicious.
Magpie. You won't believe how much harmony two voices can produce until you hear them. Folk music mostly on environmental and social justice themes, with some Native American influences. Some of the best music to be found in the generic folk/singer/songwriter arena.