it really depends on how big your tank is on whether a snail is a good idea or not. cjay is right, they do poop a lot, but they're cute and they can help control algae. you should supplement them with fresh veggies, and algae wafers if you think they need them. don't want to accidentally starve them because you think they're getting enough food with algae! also if your water is soft, you should find a way to supplement their calcium. i put little pieces of cuttlebone in my tank, and pull it out every week or two and replace it.
Moss balls "compete" for algae. They're a type of algae that actually compete with the bad algae for food (the sun).. It doesn't really work unless you have a thousand because they're not really mobile. (and my marimo moss tied onto my driftwood didn't stop regular algae- there were both kinds!) I let my pond snails clean up my driftwood nice :)
Marimo isn't an ammonia/algae solution though :)
djembah, I think you just answered a question I asked in another thread...can you tell me here or in that thread more about the cuttles? What kind, how much do you put in the tank...I worry that my two that didn't make it were not getting enough calcium for their shells, they seemed much more delicate/crumbly when they died than when I got them.