I love to listen to those 80's classic rock bands that everybody forgot about here in Europe, bands that were often pretty big in the U.S., but never broke through here for some reason.
Also great cheezy classic rock bands from the 80's that were so over-the-top that they are fantastic to see/hear! Some quick examples would be the bands called TigerTailz, Black 'n Blue, Femme Fatale, Princess Pang, Silent Rage and Pretty Maids!
-HORSE the Band
-The Bunny The Bear
-The Urge
-The Dead Milkmen
-Dir En Grey (not so popular in America)
-Flobots
-Scars on Broadway
Depends on what your view of unpopular is...apparently no one I know personally has heard of a lot of the bands I listen to so if I was to go by that my list would be a bit larger
I listen to Illnath and Amon Amarth, but those are unpopular in the sense of nobody knows who they are and not in the sense of most everyone hates them (like Bieber, Rebecca Black etc.).