Higurashi no Naku Koro ni/Kai (When they Cry 1 - 2): An awesome mystery and character unraveling, which weaves genres together effortlessly.
Excel saga and Puni Puni Poemy: Perhaps more old than obscure, but still a very fun watch.
Spice & Wolf: (Again, perhaps not obscure, but neither huge. A whole other take on the medieval genre. Calm, beautiful art, not afraid to challenge viewers mentally, and some of the best characters and interaction I've ever seen).
Elfen Lied: Good old bloody fun, and a "love conquerors all"-story that actually has something to conqueror. Word of it may have spread so far that it's not really obscure though.