Probably only teeny guys like guppies and tetras, but if a betta goes in there, be careful what you put with it. I reccommend a max of 5 fish if all fish are 1inch full grown. It's still a little much in my opinion, but many people say 1in of fish per gallon.
I usually house 1 or two bettas (divided of course) in a 5gal. The more room the better, and the less fish in one tank, the healthier and cleaner they are.