What size tank are we talking about here?
These guys have always been a big hit for me:
Tropical Fish for Freshwater Aquariums: Red/Blue Columbian Tetra
The link says they only get to 2.5 inches, but I find that they will get a little larger and quite bulky for their size.
Diamond tetras are also a new favorite of mine. They don't have a lot of colors and don't show well in the pet store, but they really are a beautiful fish. Honestly, they are not something I would have thought I would like, but after having been given some by a friend I have found that they are really a neat fish.