The wiki page https://en.bitcoin.it/wiki/Contracts has a good overview. The page https://en.bitcoin.it/wiki/OP_CHECKSIG also has a good overview of the different types of signature.

And yes, I think some of these features are currently disabled in the client. Hopefully they will be activated when the need for them arises though, and since they are already part of the protocol, it would probably be easier than making a new addition.