I have a freshwater clam/mussel (not 100% sure what the difference is) that I really enjoy. I actually pulled him out of a nearby pond and he's been in my tank for the past 4 months or so. Note that they need not only sand as a substrate, but very fine-grained sand (I'd recommend play sand you find at Home Depot, but make sure to clean it very well). They like to bury themselves, and if given enough substrate will often cover themselves almost completely.
For feeding, I just crush up fish flakes into a powder and drop it into my tank. Almost instantly the clam's siphon emerges and he begins feeding. Sometimes it's good to lift up decorations and such to stir up any debris as well.
I don't know why these aren't kept very commonly, they are relatively easy and really an interesting critter to own...