Swiki was and still is one of the best (and most easy to use/maintain) implementations of a wiki. It is written in Squeak Smalltalk and I usually use the Winterlong release back from 2005. With it a Swiki server is easy to setup (just unzip and run) and as it stores the files in XML they are easy to backup, repair or query.
Now Chris got Swiki working in a recent Squeak 4.5. image also. Nice!