[wmf]

Let's go to the source: https://en.bitcoin.it/wiki/Script "Note that there is a small number of standard script forms that are relayed from node to node; non-standard scripts are accepted if they are in a block, but nodes will not relay them."

It seems there's enough flexibility left to do some interesting things. I see discussions in the forum of things like M-of-N, nLockTime, and anyonecanpay.