Here are some that might not work for everyone (or are not 'traditionally' supposed to work), but work just fine with me. Bettas are highly individual and have unique personalities. Mine happens to be quite docile.
-Ghost shrimp (try to get larger ones as some bettas will eat the small ones)
-Amano shrimp (I have never tried these b/c they are too expensive for my taste, but if ghost shrimp work, Amanos should, too)
-Male guppies (some people say their male bettas confuse the flowing fins with those of another male betta, but my guppy and betta get along great)
-Female bettas (I have a male and female in together in a 12 gallon and both are quite happy-- I also have a fair amount of plant cover and hiding spaces in case one tries to get too aggressive)