Corys are excellent and thumbs up -- I've done both rainbow shark and tiger barbs in with betta's and have also heard of others who have. It is really a total hit and miss depending on the betta and the other fish. Though my experience with tiger barbs is not at all favorable. Rainbow shark though? He did great with my betta after one initial scuffle.
Male Bettas: Jasper (PK), Gold Dust (PK) Iced Steel (EE HM)
Other: Faith, Frankie, Sonic, Ashley & Anna (Juli cory catfish), Mr. Bristles (albino bristlenose pleco), The Shrimples (dozen ghost shrimp)