Cactus

A block which appears in sand landscapes such as desert islands. They appear in 1-, 2-, and 3-block tall columns. Cactus randomly spawn when a map is generated and when harvested, have a small chance to produce two blocks, thereby allowing the player to create more cacti then were originally spawned.

Colliding with cactus from the side has no effect, but clicking on cactus with either mouse button, even while wielding a weapon or tool, causes one half heart of damage. This can be avoided if you start mining on a different block, then quickly switch to mining the cactus. One half heart of damage is also taken for each cube of distance traveled while standing on cactus, even if just moving around on the same 1x1 block; this also applies to mobs. Cactus does not require dirt, sand, or light in order to exist.

One especially effective strategy is to create a hallway with cactus floor which you can get above (jumping puzzle, ladder, etc) and lead monsters into it. This will kill the monsters if you lead them back and forth across it. Side-view cross section:

DD D D=dirt DD D C=cactus D D  <- this opening also allows spiders to find a quick death DDCD