A quick list off the top of my head: pygmy cories (groups of 6 or more), harlequin rasboras (groups of 6 or more), shrimp (ghost or cherry), some people have luck with neons and glolight tetras, danios
Cory cats are some of the safest options, but it really depends on the temperament of your betta. One of my betta's ignores shrimp, the other eats them. Regardless of what you add, be prepared to separate the betta or take the tank mates back in case aggression becomes an issue.
Here are a few links from previous discussions:
http://www.bettafish.com/showthread.php?t=49002
http://www.bettafish.com/showthread.php?t=47610
http://www.bettafish.com/showthread.php?t=45136