Wikipedia (http://www.wikipedia.org/) describes itself as the free-content encyclopedia that anyone can edit. Yup, you heard right, anyone can edit it. That means you can edit it.
Wikipedia is based on the concept that collectively we know more about a subject than a few experts. So, if you happen to know a lot about fly fishing, find the page and add to or change the information you find. All you have to do is click the "edit this page" tab at the top of the page and make your changes. Click "save page" and your changes will appear to the next viewer of the article.
There are more than 1.6 million articles in 200 different languages (about 100 of which are active). There are more than 600,000 articles in the English version alone. Wikipedia even has a method for dealing with vandalism of its pages. A history tab enables users to revert a page to a previous version if obvious vanadalism has occured.
Try it youself and have fun!