Very interesting question that has lots of useful links on persistent object / value stores that are similar to memcache. Answer seems to be couchbase.
This says it all, couchbase includes memcached as part of it, and can operate in "pure memcache" mode or can add persistence and other features behind memcached. Sounds like it'll do what we're after.
Very detailed article explaining how Magento implements its caching mechanism. It seems to be some custom classes layered on top of Zend's caching framework. Useful read.
Very useful article on offline web apps using HTML5 cache manifests.
The WordPress WP-Cache2 caching plugin / platform. The plugin caches WordPress to static files and then serves those files instead of rebuilding the pages on every request.