The Wikipedia is a free online encyclopedia that is collaboratively written by thousands of volunteers. The site is currently is currently written in 195 different languages, has 1.5 million articles (500k in English, 200k in German, 100k in French, 100 in Japanese, etc). It's one of the most-used reference sites on the internet these days.
Given the open-source nature of Wikipedia, one would think the quality would be rather poor. But Wikipedia excels at quality. For example, check out this Wikipedia search on "search engines". You'll note the warning to the collaborators that any postings or editings have to conform to a "neutral point of view" policy. The care the editors give to this posting ensures the quality of the "search engine" encyclopedia posting.
Of course, I'm sure you can find some biased listings, but overall the Wikipedia is a great reference. For other useful references, check out our Reference category.