A school of smaller tetras or rasboras are a common addition to 10 gallon aquariums, such as neon tetras. These fish are happiest in schools of 6+; anywhere from 6-10 would work, depending on other tank mates. Another common addition is a small group of livebearing fish like guppies, mollies, or platies. All of these fish would be comfortable in groups of 4-6 in a 10gal. I'm not sure which bottom feeders are recommended for a tank of this size; I'm pretty sure corys prefer larger tanks.
I keep a pair of dwarf gouramis in my 10gal (along with 7 neon tetras) and although I've heard they prefer larger tanks mine seem happy. They're an extremely friendly personable fish, I couldn't ask for much more in such a small tank. No more than one male in a tank this size, however, as they can be territorial.