memcached: a distributed memory object caching system has APIs in Perl, Python, PHP, Ruby, C and Java and is designed for websites that have heavy database-intensive, mostly-read pages. The interesting thing here is that memcached does not exist through some theoretical “what if?” premature optimization. It was created by the LiveJournal folks who were dealing with 20 million dynamic page views per day. Other high-traffic sites like Slashdot and WikiPedia are users.
(Discovered via Titus Brown.
Update (4⁄8): memcached has been forked, apparently to fix some bugs. The new project is called Tummy.