This is a list of hacks I've made for MediaWiki. Most of them are not on a public wiki. For details, please contact me.

eAccelerator support
use eAccelerator for object cache. Just copied TurckBagOStuff and made eAccelBagOStuff. (Now in CVS.)
super-pretty links
Have effectively removed index.php from ever being seen through normal means. URLs in the form of Special:Search?search=foo. so if you have /wiki/* and /w/index.php/*, the /wiki/* method is always used.
Media URLs for real file
Requests for namespace Media (Namespace -2) redirects (using HTTP 302 Found, through OutputPage::redirect()) to the actual file.
Also hacked up MonoBook so that the markup supports mutliple visual bodies, with tabs for each and everything. Note that it does not yet work with Internet Explorer, and it does not support many features of MonoBook. If you are interested in fixing this, let me know!