The best online programming documentation I have ever seen is published by the PHP developers at PHP.Net. Each page documenting a language feature includes a message board where programmers can offer tips, advice, and workarounds (example: the file() function page).
Also, if you type a URL that doesn't exist on the site, such as http://www.php.net/sorting, a search is launched using the path of the URL as the search term.
As a PHP developer, I love this site. As a computer book author, I'm glad this level of free online support isn't common.