While Wikipedia is not a web directory, external links are allowed where appropriate. They generally are listed under a == level 2 heading == called "External links" (or "External link" if there is just one). The best way to create such a link is to type "[URL link title]". For example, [http://www.wikibooks.org Wikibooks] will become Wikibooks. However, such short link titles generally are frowned upon. Try to describe your links, so that the reader has a pretty good idea where it will take them before clicking it. Don't use external links where Wikipedia links exist. For example, in an article about Wikimedia, a link to Wikibooks (the Wikipedia article about the project) would be more appropriate than a direct link to Wikibooks.org.
Read more: Wikipedia:External links