amCharts or Quick-Charts is free, open source (licensed under MS-PL) charting controls for WPF & Silverlight. amCharts is a robust charting tool that will suit any dataviz need. Charting solution include Column, Bar, Line, Area, Step, Smoothed line, Candlestick, OHLC, Pie/Donut, Radar/Polar, XY/Scatter/Bubble charts as well as advanced Stock charts.
D3.js – Data-Driven Documents
Exhibit – Publishing Framework for Data-Rich Interactive Web Pages, Exhibit lets you easily create web pages with advanced text search and filtering functionalities, with interactive maps, timelines, and other visualizations.
jqPlot Charts and Graphs for jQuery
Protovis – A graphical approach to visualization
Timeplot Web Widget for Plotting Time Series
Timeplot is a DHTML-based AJAXy widget for plotting time series and overlay time-based events over them (with the same data formats that Timeline supports