Wikipedia is an free online encyclopedia that anyone can edit.
From Wikipedia's About page:
Wikipedia is a multilingual, web-based, free-content encyclopedia project based on an openly editable model...Wikipedia is written collaboratively by largely anonymous Internet volunteers who write without pay. Anyone with Internet access can write and make changes to Wikipedia articles (except in certain cases where editing is restricted to prevent disruption or vandalism). Users can contribute anonymously, under a pseudonym, or with their real identity, if they choose.