Here's some info I found:

Bettas are actually very friendly fish as long as you don't put two males together. They get along with most other fish, and what you really have to watch out for is the fish that harass bettas.

Bettas are slow moving fish with very long fins, so they can't be kept with any quick fish that have a reputation of fin-nipping, like zebra danios. They get along *great* with most variety of tetras, and I raise my baby fish with baby neons all the time. Mollies and platies and other of that variety also work well.

I've had bad experiences with keeping bettas and guppies, as the male bettas will mistake a male guppy tail for its own sort and fight. Other people do fine with them, though.

No goldfish. Goldfish come from cold water and bettas from very warm water, so their temperature requirements are very far apart, and also their general water conditions. Goldfish are very heavy water pollutants and require an absolute minimum of an inch and a half of fish per gallon of water (the usual way to estimate how many fish in a tank is one inch per gallon).

When mixing bettas in a community tank, the thing to keep in mind with bettas is that they are fish that originally came from very an area with slow moving water. So any type of major filtration on the tank will shock a betta and make him hide in a corner away from the current. You have to balance this with the other fish in the tank that require the current and bubbles for the oxygen in the water for them to live. On the other hand, once a betta gets used to the "current" from a filter, you will sometimes see the betta 'playing' in it for fun. Just make sure that there is a quiet area in the tank for the betta to move to when they are tired.