There's a few varieties of snails, ramhorns, MTS (Malaysian Trumpet Snails), and the traditional pond snails. If it's a small tank, I'd get some MTS since they're prettier than the other ones. The shell looks like a twisted icecream cone I guess you could say.
The snails will go after the algae and would benefit from a lettuce leaf or algae flakes/wafers now and then and won't hurt any plants or your fish in the aquarium.
Oh, and another word about snails, I hear they reproduce like mad. I just got a few for my starter culture, so we'll see how fast they grow! =)