Hmm - online whiteboards always seem like a good idea to me, but I have never been able to sell them.
I like this one because
1) it requires no sign -up and
2)it lets you take a snapshot of the page. Maybe this would work with math notes?
A collection of resources and lesson plans based on teaching history from a passionate place, a nontraditional view, and a social justice view of history.
Classroom idea: Lots of ideas for lessons that could go cross curricular - especially religion and history.