A relatively simple trap where NPC's are killed by cacti. The NPC is repeatedly flushed against the cactus by the water.

Pros:
-No Redstone necessary
-not much space needed for actual trap
-inlet can be extended as required
-trap is not destroyed
Disadvantages:
-cactus needed

Instructions:

Picture is self-explanatory.
In front of the cactus is a funnel connected to a chest.