wikipedia.org provides a collection of free wiki-based encyclopedias in many different languages. References to articles under wikipedia.org in fact redirect to the English encyclopedia at en.wikipedia.org.
The Wikipedia publishes under the GNU Free Distribution License, whereas SL publishes under the Open Content License. Although this difference could cause copyright issues, both licenses have the same goal: to enable and encourage copying and reuse. Hence, there should not be major issues when one complies with either of the licenses.
See the /Copy Discussion, giving rise to the above section.
WikiMedia is the organisation that runs Wikipedia.
WikiMedia Commons is the central collection of media used in multiple Wikipedias.
For a collection of Wiki websites, see Go Wiki Wiki
Also see the list of mindsport wiki for wiki's on other mind sports