I am a big fan of Wikipedia, using it extensively as a way to provide depth to my online writings. Although I have made minor edits (e.g., spelling corrections) in the past, I have only recently begun to make more substantial contributions. These are typically motivated by the fact that I wanted to link to a Wikipedia entry, but could not find one.
Aside from being a useful tool, Wikipedia is very interesting as an experiment in social software and (prospectively) knowledge engineering. Thus, I am quite intrigued by the efforts to extend it into a Semantic Wiki, as documented on the Semantic Wiki State Of The Art page on the Semantic Wiki Interest Group web site.
I am currently working on a specification for Ontiki, an "ontology-aware" wiki which I hope will be useful in Model-based Documentation. See my home page and resume for general and professional information.
In progress: /Protege