Apistogrammas! Any kind, borelli, cacatuoides (which mine are the coolest), agasazzi... those are the only names i can spell of the top of my head. They get pretty difficult the farther down the list you go. a fifty gallon could sport a nice little family! Here's some pics
http://www27.brinkster.com/keprydak
there are of my own stunnning pair

(yes I am very proud)