Calling for all links!

I'm looking for all useful links to resources, books, and games for learning. Free is the operative word. User friendly is nice, too. Over the years, I've gathered quite a few links and they are currently spread out over four or more computers. I am going to attempt to tame them by listing them on this blog so they will be available to anyone who has use of them. Feel free to leave links in the comments and I will get them added. Thanks! This will be left as a sticky post for awhile.