What are useful online tools for Linux

As you know, GNU Linux is much more than just an OS. There is literally a whole sphere on the Internet dedicated to the penguin OS. If you read this post, you are probably inclined towards reading about Linux online. Among all the pages that you can find on the subject, there are a couple of websites that every Linux adventurer should have in his bookmarks. These websites are more than just tutorials or reviews. They are real tools that you can access from anywhere and share with everyone.