Via freshmeat, PmWiki is a PHP-based Wiki with a clean but nice look to it. I really like the editable sidebar. Also, if you add a URL of a .gif, .jpg, or .png, it’ll display the image on the page. Pretty slick. The basic syntax looks good, though as with every other wiki on the plant, it’s just different enough to leave you constantly scratching your head.
“which wiki planet am I on right now?”
Update: Wow. I decided to install PmWiki. I was thoroughly impressed with how lightweight it is (no mysql neccesary) and how easy it was to set up. I followed the installation instructions and went from a tarball to a working wiki in less than 60 seconds.