Wakka, a lightweight WikiEngine powered by PHP and MySQL. Wakka is still a child, so feel free to check out the documents listed there to get an idea what's in, and what's still to come. Wakka, by the way, is being developed by Hendrik Mans and Carlo Zottmann. It's named after a character from the video game Final Fantasy X.
This wiki uses a modified version of WakkaWiki. You may download the modified version at WikiDev.