Via Scott Chamberlain: A species of ants that lives in and around carnivorous pitcher plants isn't entirely freeloading. They also clean the walls of the plant's pitfall trap, keeping it nice and slippery for insect-trapping.
Of course, the ants have a vested interest in keeping the trap effective, since they eat some portion of the critters caught by their host. But it seems pretty straightforward to think that this helps the pitcher plant, too. A more definitive test would be to compare the survival and seed production of pitcher plants grown with and without a colony of ants to keep them clean. ◼