The standard response is that they do fine with other fish that aren't nippy (i.e. tiger barbs, serpae tetras, and plenty of others) and fish that aren't overly-flashy (male guppies) or closely related (other betta species) or both (dwarf gouramis). Also avoid fish so tiny the betta could eat them, like neons.
That's the standard response, but it also depends greatly on the individual fish. Some bettas really don't tolerate any sort of tankmates at all, while some don't seem to bother any other fish at all.
Given a large enough tank, you could certainly try housing the betta with other fish (although I'd try to stick to the guidelines in the standard response), but be prepared to move him should problems arise.