This site from Drexel University was one of the earliest comprehensive math sites. It has many resources for teachers and students. This particular page is for teachers, but there are student pages, "What's New" pages, and more.
Very comprehensive site that has many useful features, including daily podcast booktalks. I often suggest the site to teachers looking for books of a particular genre, theme, etc. Some of her pages that link to this are now wikis.
One of the best, most comprehensive resources for free materials available online. Check out the live events page! This is a collaboration among several organizations.
Very comprehensive article with an extensive bibliography. Allow some time to thoroughly digest it. More needs to be shared about the joys of working with middle schoolers