I contribute/contributed to the following wikis:
List of other personal wikis I've come across. Unfortunately very software-centric.
I tend to write my notes in Markdown (may just switch to Org at some point?), so being able to easily convert from that syntax to something I can plonk into a wiki is very useful! You can even grab HTML files and stuff straight from the WWW.
☞ I don't like Markdown enough to actually use it as the markup language for the wiki, though.
MD to Doku
pandoc [inputfile.md] -f markdown -t dokuwiki -o [outputfile]