I just got this off a website..
Safe Tank Mates
Because of their attractive fins, male bettas are often the targets of much bullying, like angelfish if you've kept them. For this reason they are often kept by themselves. They do fine this way, but if you'd rather keep your fish in a community tank, you do have some options. First, no two male bettas should ever be kept together. They will fight, and be in a constant state of stress. Females may be kept together or in a tank with most any community fishes, but provide plenty of hiding spots, as they have been known to fight, too. A male and female betta shouldn't be kept together except in a breeding situation or they will likely also fight. Suitable tank mates are non-aggressive and non-territorial, and must need the same tropical water conditions as the betta. Keep in mind that most fish will need more aeration in the water than bettas. It may take some work to find a happy medium for all your fish. Try guppies, mollies, or any of the tetras. Algae eaters should be avoided unless necessary, as they may try to "eat" off your betta's fins. You may also consider other aquatic animals such as snails or African dwarf frogs. Regardless of how gentle a fish you buy to go with your betta, be on the lookout for any rogue fin-nippers and be prepared to move them to a separate tank. If your betta's fins appear ragged or broken soon after a new addition, take action to remove the offender and keep the water clean to prevent infection.
http://www.klsnet.com/betta.html