Aren't there two p's in "a cappella"? That's how my ancient Webster's lists it.

My trad folk faves have already been mentioned (and good on ya for finally mentioning the Silly Sisters albums). So I'll take the opportunity to list a few a cappella notables a bit farther afield:

Uncle Bonsai, an acoustic trio that escapes description, though "sardonic" would have to be in there somewhere. Their harmonies were tight and unconventional, and they could sing a patter song like Gilbert and Sullivan on speed.

The Blenders, a fantastic doo-wop group out of Minnesota. Kinda like Rockapella meets The Bobs.

The first two albums by Take Six (gospel/jazz). This is jaw-dropping a cappella singing.

Todd Rundgren's A Cappella--he sang everything on the record, including the "instruments". True to his chameleonic talents, the styles run the gamut, including a song in the traditional English folk idiom, sung mostly as a vocal solo.

At the other end of the spectrum, David Hykes harmonized with himself in the literal sense, by producing distinct harmonic overtones. He also put together a "harmonic choir"--an eerie listening experience.