An interest group that shares valuable tools/conversations/ideas for developing mathematical understanding naturally (not just in the classroom). Maria Droujkova is one of the driving forces behind the group, and she's brilliant and dedicated to the cause.
Interesting games for students to play online. A free version of the site is available, but the real value would be in using the purchase performance package if you are going to use it routinely in your classroom.
Is this the direction textbooks will take in the near future? The content is important and is the center of this idea. It's also slightly portable (downloadable, accessible on up t o3 computers, and able to print 30%). This site makes annotations possible and more importantly sharable! The content is supported by learning communities! I like this idea; you even have the possibility of sharing beyond your classroom walls.