I learned from Emily Chang's blog about a new online data analysis/visualization web application called Swivel. I loaded some fake student performance data to see how usefull this might be to teachers. If you want to see the datasets I loaded search for "millerjtx".
Swivel has potential, but is too early in development to be truly useful to the time-crunched classroom teacher. However, the potential for collaborative exchange and discussion of data and visualization is encouraging.