I saw this on the Harry Potter spot...
The rules are so simple. I put a coin in and the next poster tells me what I got, and then puts a coin in.

For example:

MrsLunatic: *Insert coin*.
Someone: You get a key of the TARDIS *Insert coin*.
Someone else: You get a cyberman's head *Insert coin*.

Get it? Yay!:D

*Inserts coin*