Wikipedia - Online Wiki Encyclopedia
Wikipedia is a free online service and a web-based encyclopedia which volunteer users can collaborate and edit articles. This online encyclopedia is always up to date within minutes. It is available in multiple languages with almost 4 million articles and 27 million pages in total.
Anyone can find huge amount of information about anything. Usually when you search for something on search engines, Wikipedia's articles is on the top searches. The main advantage of Wikipedia over printed encyclopedias is it is always updated and of course it is free. Its weakness is it's credibility although administrators instantly remove vandalism on their articles.