Created in 2002, PmWiki is one of the oldest wiki engines still actively developed, at 15 years old as of this writing. It's written in PHP, and as such requires a properly configured web host (some are simply broken), but no database. Installation requires the knowledge to edit a config file and set directory permissions via FTP, but otherwise is quick.
More important are the features that set it apart:
- A variety of ways to link pages, such that the text looks natural yet still points at the same canonical URL.
- Page trails with automatic previous/top/next links.
- Ships with a complete set of documentation; each and every install is self-documented.
- Very easy to make skins -- you only need one template file containing ordinary HTML interspersed with placeholders.
- Despite all that, PmWiki is still only 1.4MB in size.
- While pages are stored in flat files, the data format is opaque; you pretty much need a working PmWiki install to retrieve the text.