A pair of cories (if you could find pygmy cories you could go with even 3 or 4)? A couple african dwarf frogs? Some fresh water shrimp? A couple small tetras (like neons)?
There's lots of choices! You want him to be the biggest fish in the tank and you want to avoid fish that nip at tails or that have long flowy tails cause he'll perseive them as a threat (so no guppies). And usually you want something small that will back down if the male betta flares up at all.
I find Betta's go really nicely with a bottom dwelling type since they tend to hang out on differant levels of the tank you tend to have fewer territory issues.
Frogs and Cories in my experience seem to go really well with bettas because they have similar diets so they'll pick up what ever the betta doesn't eat and have similar preferances in water and environment.
Just make sure you give some hiding places for what ever you pick.