Nothing too crazy or aggressive. I would recommend cories, platies, mollies, or the smaller tetras (neons, cardinals, von rios, bloodfins). The larger tetras (like black skirts, black phantoms, etc) are likely to fin nip unless kept in VERY large school, but even then they might find a betta's long, flowing fins too much to resist. Danios are too energetic for bettas and are potential fin-nippers. Same thing goes for guppies. That is a great link Bristlenose provided you with.
To be completely honest, I would just get your betta his own nice little 5gal tank (heated, filtered and cycled, of course) and leave him by himself. I had my son's betta in his 29gal community for a while and he was just WAY too stressed, even though everyone left him alone. He was in with platies, guppies, a couple of oto cats, and an upside down catfish.