skrbl is a free web-based whiteboard. All you have to do is start skrbl and give out your URL ( here’s one of mine: http://www.skrbl.com/49043427) in order to instantly begin sharing your board online. What a great collaboration tool!
If students purchased a Wacom tablet for home use (a 4×5 is under $100), I could see real possibilities next school year for homework collaboration and online study sessions online. My students use an electronic whiteboard or graphics tablet in class to create their own mathcasts, so they already know how to use this technology.