Some examples are professional journals like Mathematic Teacher and Teaching Mathematics in the Middle School. There are lesson plans, technology tips and information about professional development. It includes articles, games, puzzles, and research topics for you and your students. It also has links to many other sites. It has 5 problems of the week that are designed for algebra, middle school, elementary and geometry.
Students receive feedback and their names are posted on the site if they are correct.
There are also archived problems from the entire history of the site. This site is maintained my the University of Saint Andrews in Scotlans. It has biographies of mathematicians indexed several different ways. It has a search feature and many other links.