Mastercoin is described as:
"both a new type of currency (MSC) and a platform. It is a new protocol layer running on top of Bitcoin like HTTP runs on top of TCP/IP. Its purpose is to build upon the core Bitcoin protocol and add new features".
Are there any other protocols or similar built on top of the Bitcoin protocol in this way? (As an aside, the way Namecoin uses a Bitcoin-like P2P model to act as an alternative, decentralized DNS is interesting...are there similar things built on top of Bitcoin itself?)