We all know Wikipedia and it is used a lot in this blog. Everybody is able to write a contribution to this on line encyclopdia and to consult it free of charge. There is a limited peer review to avoid that complete nonsense would be published. I consider it as the best contributing social medium and I can “loose” a lot of time by just checking the main page.
There are also other projects in the same format like wikibooks, wikispecies, wiktionary, … . They are all based on the same open source software of Mediawiki.
This software is written in PHP and works together with a (open source) database. But is less known that this software can run on your personal or company server where you can develop your own documentation system. Indeed, a PC under Linux can run this software for your private documentation system at home or in the company. BELGLAS can assist you in starting such a documentation system where each document gets its own comments in order to find it when you need it (when you already forgot you made it). The software is free of charge like all open source.