Let's see, neons are always a favorite.
There's different types of rainbows, I'm partial to the threadfins and furcatas.
Peacock gudgeons
Badis badis (aka scarlet dario or dario dario)
various loaches (partial to kuhlis and clowns for color reasons)
cichlids, darn near most are brightly colored
What size tank and what look are you going with, natural vs unnatural?