If a player or mob walks over an Anenemy, it will trigger and deal 60 damage to anything on top of it and close itself for around 1 minute. The Anenemy has unlimited durability, but unlike the Tooth Trap the player cannot reset it.
Closed Anenemies can be dug up with a Shovel, giving an Anenemy Trap, which can be picked up and replanted elsewhere, spawning a closed Anenemy at the chosen spot.
The Anenemy can not be set on fire so it is a great way to kill Fire Hounds.
Anenemies are excellent at mass producing Spider drops. Placing multiple Anenemies around a Spider Den will kill multiple spiders. Provided the player is capable of extracing the drops from the Anenemies, one can gather lifetime supply of silk doing minimal amount of work.
Combine this with Bunnymen, walls and perhaps some bait (along with clever placement), you can create a fully automatic Bunnyman farm.