Well, take the following into consideration when searching for fish to be with a betta.
1. Make sure the kind of fish is NOT bright colors - Bettas will attack other brightly colored fish such as guppies and some tetras
2. Make sure the fish you want is NOT nippy, as a bettas long, flowing fins can be appetising
3. Check if the fish needs to school or not, and if it can be in a tank your size.
I suggest, with that information, looking around the fish index pages here. I keep by betta in a 20 US gallon tank with 7 bloodfin tetras, 2 Dojo loaches, and one oto, and have never had a problem. But remember - It is never SAFE to keep a male betta with other fish... There are just SLIGHTLY safer methods then others. Your best bet is to stick to bottom and midlevel dwelling fish, as fish who stay at the top might bother the top-dwelling betta. Cories are good, as are otos and some tetras (just be careful they are not nippy ones)
Hope this helps,
Katy