TOTD:
I tend to go more for instrumental groups than vocals..
American: I tend to like songs more than artists but I do tend to favour Peter, Paul and Mary and Chuch Suchy. Suchy is a regional artist. I also enjoy bluegrass. I also like Los Lobos when they used to do more of the Mexican root music.
Ben, I also enjoy the McGarrigle sisters. Excellent singers.
Metis and Canadian: La Bottine Souriante (they fuse Jazz with French-Canadian trad songs - feiry and fun to listen too) and Alex Carriere
American/Canadian Celtic: Rankin Family, Loreena McKennit, Cherish the Ladies, Great Big Sea (Excellent singers of sea shanties), Natalie MacMaster, Ashley MacIsaac, Eileen McIvers, Reeltime, Anne Heymann, and Black 47 (they fused pop and rock with Irish trad music)
Galician (Spain) : Carlos Nunez and Luz Cabal (beautiful singer)
English / Scottish: Capercaillie (Karen Matheson has an incredible haunting voice and it goes right through you.), Robin Williamson, Talitha McKenzie, Fairport Convention, Silly Wizard, Tannahill Weavers, Boys of the Loch, and the Battlefield Band
Breton (France): Kornog
Norway: Sissel (She started out as a folk singer and is very popular up here.)
and of course Irish Trad/Folk Music...
My all-time favorite is The Chieftains!
I also like Altan (Mairead has a beautiful voice), Clannad, Anuna, Mary Black, Michael O Suillebhean, Matt Molloy, Derek Bell (He has recorded innumerable harp albums and classical music), Bothy Band, Kevin Burke, Tommy Potts, Sean O Riada and Ceoltoiri Cualann, Davy Spillane, DeDanaan, Deanta, Dubliners, Clancy Brothers and Tommy Makem, Seamus Ennis (wonderful piper and story teller. I am glad the Gael-Linn label is re-releasing his stuff again)
Those are the ones I can think of right now.....