IBM's Many Eyes was a pioneer in the Web-based visualization space. It's well-known and very versatile, with 20 or so visualization types such as plots, bar charts, maps and network diagrams.
The Many Eyes project goal is to encourage people to share and analyze each other's data. That makes it easy to use, but it also has limits for sensitive projects: In order to use it, you must upload data to the project's site and make it publicly available.
Find it at: http://www-958.ibm.com/software/analytics/manyeyes/