The Nexista Cache Extension is not very well documented but it is a pretty sweet resource. It uses PEAR's Cache_Lite package to cache output.
It supports the following cache mechanisms: * Expires - so the client will maintain freshness without checking for a certain period of time * 403 headers for non-modified content * User-specific and private caches
So there are really two parts to Nexista Cache, the headers, and the cache storage and management process.