One of the best free learning resources on the Web is Wikipedia. It has been built by thousands of altruistic volunteers from across the world. Despite some controversies, it is favored by millions and it runs on donations. If you have benefited from it, you can donate to the cause or pay it forward by contributing your knowledge and time to it.Editing existing articles or adding new ones though is not for the technically faint of heart. Luckily there is now help at hand. The how-to book Wikipedia: The Missing Manual is now available online for free on Wikipedia. (Thanks Amit)
Update(27-Apr-09): You can create a book out of any content on Wikipedia and download the finished book as a PDF.
Talking of donations, Philip Greenspun gifted $20,000 for creation of illustrations for Wikimedia Foundation projects.
"It occurred to me that when the dust settled on the Wikipedia versus Britannica question, the likely conclusion would be 'Wikipedia is more up to date; Britannica has better illustrations.'"
By the way, the popularity of Wikipedia has been a cause of serious concern for Encyclopedia Britannica which is now inviting the hoi polloi to edit, enhance and contribute to its online version. It's CEO is displeased that Google ignores it in favor of Wikipedia. I feel it is because the Britannica website is not optimized for search engines. Healthy competition is good though and Britannica does not appear to be sparing any efforts to catch up.
Related: HOW TO use Wikipedia content in your application