WP Super Cache is a fork of the popular WordPress plugin WP-Cache 2 that, apparently, does an even better job of caching than WP-Cache 2. According to the WP Super Cache site,
WP-Cache 2 caches the pages of your WordPress blog and delivers them without accessing the database. Unfortunately it still means loading the PHP engine to serve the cached files.
WP Super Cache gets around that. When it is installed, html files are generated and they are served without ever invoking a single line of PHP. How fast can your site serve graphic files? That’s (almost) as fast it will be able to serve these cached files. If your site is struggling to cope with the daily number of visitors, or if your site appears on Digg.com, Slashdot or any other popular site then this plugin is for you.
Given the spaghetti code that is WordPress, it is a bit mind boggling that something like this hasn’t been made part of the WP distro already. I know something like it is currently being planned, but WordPress can’t even withstand moderate traffic without noticeable performance hits if you’re not using some sort of caching plugin.