This week, two mods for Left 4 Dead:
One which mimics Super Mario 64 settings, in case you felt Mario didn't have enough zombies to fight.
The other replaces zombies with Teletubbies, in case you want to do violence to children's TV.
Mokalus of Borg
PS - I haven't actually played Left 4 Dead, modded or not.
PPS - I don't know whether I would enjoy it.