For a 10 gallon, I really like small tetras, if the specific sort of tetra isn't known for fin-nipping. :)
Otocinclus are SUPER cute, in my opinion, but they eat algae and such, so you'd have to drop wafers in for them or some sliced vegetables. Otos are also a schooling fish that are happiest with at least 3-5.
Smaller species of peaceful rainbows are nice, too, like the furcata. You can probably put 3-5 of them in there with the betta. They put on quite a display with their fins, and they don't nip. You'll need to be sure you feed them bits that are small enough for their tiny mouths, though. They're one of my favorite fish.
It really all comes down to the betta as well. I have two bettas that are thriving in community tanks. One rules over a 15 gallon with plants, otocinclus, and furcata rainbows. The other lives peacefully in a 27 gallon with threadfin rainbows, otocinclus, and a mild-mannered German Blue Ram.
And then I have Sashimi :P He won't even tolerate a pair of otos in his 8 gallon to keep the plants clean. He flares at any fish I put near his tank.. So he has horned nerite snails, and he's okay with those. :P Anything else, I think he'd rip apart.