For americans, I know of Convicts, Rainbow Cichlids, Cryptoheros nanoluteus and the likes. Oh and my Ellioti breed in a 15g, so Elliotis too I guess.
Note that this is breeding. These can breed rather comfortably in a 15g, but for most, it is definitely not good enough for life. And also 'can' here means that if the pair pairs up fine then good, but aggression can tear it all apart.