Twiddla is one of the coolest web apps I have seen in quite awhile. It allows you to whiteboard with team members from all over. You can set up your session - invite whomever you like, and then get to work. The functionality is really easy to work with and you can upload images and mark things up very easily. While there are other free online collaboration tool sets out there, this one is one of the easiest to use and most fun that I have run across.