Corydoras(corycatfish), zebra danios, any tetra that doesn't have long fins, rasboro. Do not place long finned fish in such as goldfish or aggressive fish such as barbs, other male bettas, gouramis or angels.
Any Fish without long fancy fins or are known "fin nippers" can go good with a Betta. . Betta's are part of the gourami family, which are known to be semi aggressive but can do fine with none aggressive fish such as Zebra Danio's, Neon Tetras or any of the platy species.
Some people keep bettas in community tanks, and some people try mating a male and female. You cannot keep two males together even in a large tank. Apparently they are just fine on their own in a small bowl or tank, too.
Betta Fish, or Japanese Fighting Fish, typically do best in an isolated environment. This is because their natural instinct is to acquire and protect their territory from other fish, which they view as threats. Betta Fish do not need, or want, companions.