Kehei Wiki, one of the oldest wiki implementations.
Targeted at commercial environments, provides adjustable enforcement of author rights, WYSIWYG editing, optional blog by page, multiple namespaces, templates for everything.
Integrated rich-text editor for those who don't speak HTML.
Classic installer runs itself from browser in about 30 seconds. Specifically designed for ISP/hosted installs (no need for shell access). Can go in a subdirectory
Commercial support available.