Smart Board Goodies is packed with ideas, links and resources to use on a Smart Interactive Whiteboard. The Whiteboard Blog is another favourite resource area, check out the post 20 Interactive Whiteboard Resources for Teachers.
Smart Exchange is a place where teachers can share whiteboard files. Promethean have a similar site called Promethean Planet . The Lighthouse for Education also contains Promethean Flipcharts.
There is a further list of Whiteboard sites here. Have I missed any?