I've almost finished rewriting Cache_Memcached which provides a PEAR module for connecting to memcached servers using a Object Orientated interface. Memcached is a high-performance, distributed memory object caching system, generic in nature, but intended for use in speeding up dynamic web applications by alleviating database load.
Cache_Memcached still needs to be profiled for see if any speed improvements can be implemented. The OO is similar to the memcache perl client which was written by Brad Fitzpatrick et. al. from Danga Interactive.
Progress Status
2005-06-27: re-added the PEAR proposal.
Various sites from Live Journal, Wikipedia, Slashdot, SourceForge and many more sites are using memcached.
Various sites utilise Cache_Memcached.