Highcharts is a great javascript framework to build all kinds of interactive data visualizations. You can see some examples below, or visit navigateobscurity.com to see more visualisations in context.
Sigma is a javascript framework that specializes in creating interactive visualisations of network graphs. It works great on its own, or in combination with other network visualisation tools like Gephi.
Tableau really shines as a desktop application as it can be used for quick data explorations and visualizations. But with a small sacrifice on performance it can also be used to embed data visualizations on the web. See some examples below.