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", even if there is only one link. The best way to create external links 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 external links, so that the reader has a pretty good idea where it will take them before clicking on it. In the body of articles, do not 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.