There is a cool project that my company (Rackspace) has opened sourced called Repose. It is a proxy for ReST APIs that, among other things, supports rate limiting in a clustered environment. It's still pretty new, but we use it internally and might be useful for people writing APIs. You can find it on github:<p><a href="https://github.com/rackspace/repose" rel="nofollow">https://github.com/rackspace/repose</a>