Quick mention for a newish Edinburgh 6-part female a-cappella group called the Wild Myrtles, specialising (like Muldoon's Picnic of Glasgow) in original arrangements of folk and world music.
http://www.myspace.com/thewildmyrtles

Further afield, Colman's Well in Vermont are superb but not currently active, due to one member becoming a parent I think.
http://www.myspace.com/colmanswellmusic

Anyone heard of Corou de Berra? French-language group (well Occitan I suppose) based in Nice. Specialising in local traditional harmonies though possibly some of the arrangements might be a bit experimental for purists.
http://www.myspace.com/coroudeberra
http://www.coroudeberra.net/