You can check the forum section of Betta fish compatibility
Otherwise, platies, sword tails, corydoras, guppies.. people use different species of the smaller, schooling tetras- just keep in mind they can be fin nippers. Not always, but sometimes.. make sure to get a group of 4-6 if you go with them.
Loaches tend to be good- but they can be a little more sensitive to water quality and parameters then your average tropical fish.
Stay away from the barbs, gouramis, cichlids, catfish (other then cories), danios for the most part.