This is another jQuery plugin, and produces results not dissimilar to Flot. It provides quite a wide range of graphing options.
Dojo Charting, part of the Dojo Toolkit, provides many common chart types including line, bar, area and pie charts.
Inspired by Flot and PlotKit, Protochart build upon Prototype and ExCanvas, providing a good range of charting options.
Google Visualizations provide a variety of ways to represent your data, including charts from the Google Chart API as well as pure client-side visualizations (some of which require Flash).
YUI charts let you visualize tabular data on a web page in a variety of different ways, but unfortunately the chart control depends on Flash 9.
This is not just restricted to charts, and it doesn’t support IE (yet), but I wanted it to include it anyway as, like Raphaël, Processing is such a powerful tool.
Please let us know of any omissions in the comments!