Some of the best ones have been...
Flux Capacitor.. mentioned before, and that never gets old.
Left handed smoke shifter. (or anything left handed) - also a good thing to make freshman go find from teams when they are bored.
Jersey Barrier (This was at the NJ regional, so that made it even more funny)
Pinecone... classic randomness right there.
I'm sure there are others I'm forgetting.
Of course, these are usually done at the end of the day on Saturday when bots are being packed up so no valuable P/A system time is used, and generally all in good fun and well appreciated by both teams and Pit Admins who have both had a long few days.