Dan Meyer writes about having teachers move away from being an "answer key" for their students to encourage more exploration and discovery.

This website has some fun games for teaching/refreshing basic math facts. There are also some more complicated games for things like fractals and calculator use, so the site could be used with a variety of classes.

The NCTM website is one everyone is familiar with; there are a lot of resources here that are great for explaining concepts. This is an interactive activity working with vectors to help students understand basic facts about the difference between magnitude and direction.