What protocols are built on top of Bitcoin? 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?)
New information: 

See David Schwartz's response at examples-of-asset-backed-virtual-currencies for a short description of Ripple.

Older information:

The List of all Cryptocoins  mentions both Mastercoin (MSC) and Ripple (XRP).  This URL is dated, and Ripple is now open sourced. The ripple video is informative.  According to Wikipedia, Ripple is currency agnostic. Ripple has network consensus times of 2 to 5 seconds (which is much less than 2.5 or 10 minutes) and already has 100 billion units of XRP which probably explains why it is not visible at Mining Revenues. I've yet to dig into the Bitcoin Ripple Open Forum.

The Central Banks are scared that an open source architecture is emerging that will be redefining the FIRE Sector, and thus threatening their monopolies.  The FIRE Sector is starting to go through a very disruptive innovation revolution with the genius of Open Source Technologies. If these distributed technologies are implemented properly, we could be on the verge of another 1776. I would love to see the diagram below updated and refined by the Open Source Community so we all have a bigger picture road map to focus our efforts.