XCache Plugin for WordPress
I was sitting here the other day saying to myself, you know, wouldn't it be nice if I could use the Var cache in XCache to eliminate some of the (far too numerous) queries used by WordPress? Well, I set myself on building a plugin to do just that, but it turns out that someone else actually already did it!
Now, before I go any further I should mention that this isn't actually a plugin, it's more of an extension of existing functionality. It doesn't go in your 'wp-content/plugins' folder (that was the first mistake I made, I never read the directions), it doesn't show up in your plugins list (making you check manually for updates, even under WordPress 2.3), and it doesn't have a pretty GUI in 'wp-admin'.
What it does do is eliminate a number of common queries that get run over and over for every single page view. On this site it eliminated a total of 8 queries per page load. While that only dropped the page generation time by about 0.1 seconds, every little bit counts.
Oh, and if you're one of those people that still doesn't use XCache (why aren't you, by the way?), the author created a version for eAccelerator as well (and links are provided on those pages to the original work for APC and memcache).
These two were just recently updated so make sure you give them a try.