Found PmWiki (when it was still an early 1.?) after discovering what a wiki was, and losing patience while trying to install things like MediaWiki, and a number of other more complex packages. Simple installation and maintenance are key factors for any application, and PmWiki scores highly.
My main use is probably personal on my own intranet at home, where it avoids the hassle of being tied to one PC. Although I have a passion for using database management systems, they're largely too formal to use for most personal application, and I find the greater freedom of groups and categories is ideal for keeping track of information that's too general to be maintained within the formal structure of a database.
It's also proven ideal for making programming notes and records of software revs.
On the web