http://docs.google.com/: No math, many shapes, shapes are hard to modify.
http://dabbleboard.com/: Has math, drawing libraries, shape detection.
http://scriblink.com/: Has math.
http://www.kerpoof.com/: Kids oriented.
http://cosketch.com/: Interesting text adding.
http://flockdraw.com/: Cute but simple.
http://awwapp.com/: Cute but simple.
Other lists:
http://www.pearltrees.com/viabf/boards-white-smart-ones/id5787731
