tuck tuck tuck and arrrrwalktheplank are two of my favourites. The first summons a monster truck in AoE3, and the second has a large pirate ship jump into the battle in Free Space 2.
Actually there is a set of missions in FS2 that can only be reached by cheating to become invincible during an event you were supposed to die I think.
I find it neat when companies put in assets and extra content most players wont find without trying to break things a bit.
He forgot to do his neopets dailies.