Wikipedia is a free, multilingual, open content encyclopedia project launched in 2001 by Jimmy Wales and Larry Sanger and operated by the non-profit Wikimedia Foundation.As of April 2008, Wikipedia had over 10 million articles in 253 languages, about a quarter of which are in English.
Wikipedia offers free copies of all available content to interested users. These databases can be used for mirroring, personal use, informal backups, or database queries (such as for Wikipedia:Maintenance). All text content is licensed under the GNU Free Documentation License (GFDL).
If you need only English content, you can fetch the English Wikipedia SQL (or rather MySQL) dump.
You can also access any article programmatically, one at a time in XML or as an RSS Feed.
This is what Answers.com possibly uses to fetch aggregated results including Wikipedia content, for user queries.