peppermint shrimp and emerald crabs
Both are fun and harmless, and do a good job of cleaning and moving stuff at the bottom of the tank.
Best thing pepermint shrimp are $3 and the crabs are $5 at my LFS
I don't like hermit crabs anymore.. the blue leg ones kill off snails and red legs way to frequently. I started with 12 red and blues, and now have 12 blues and 1 red, and lost a couple snails also. (I have empty shells in the tank also).