In the good ol’ days WordPress came with a filesystem object cache but it was removed some time ago because it was a pain to maintain, and caused problems for some users, especially those using NFS. Nowadays there is an object cache built in, but the cache only survives for as long as a page is being served.
Other developers have taken up the challenge and produced object cache plugins to fill in the gap. There are the neosmart ones including a filesystem object cache and a memcached one (Read Andy’s notes before installing).
The neosmart filesystem object cache (and the others according to #988) don’t work correctly with WordPress MU so I dug up a patched version of the filesystem object cache I worked on a year ago to look for testers.
Download object-cache.txt, rename to .php and copy into wp-content/. It should start working automatically but if you don’t see files and directories in wp-content/cache/, make sure that directory is writeable by the webserver.
The neosmart version on which this one is based doesn’t handle switching blogs at all. Cache collisions occur with data from one blog’s options polluting the options in others. The version linked above should fix that but I’d appreciate some testing by others.
Oh, check out WordPress MU trunk now. I merged WP 2.8 beta1 and I’m fixing bugs. Please install and try it out on a test server! The get_option() and related code is using the same code as WordPress.org which is one of the main reasons I went digging into the object cache. It leans a lot more on the cache than previously. Please test!