I'm trying to build a collection of good chart resources. The problem with the PHP libraries in PECL and PEAR is that they are still alpha/beta. I you have used a specific library/tool please add them to this wiki and if possible please add the experience you had with it.
pChart
http://pchart.net/
Flot (jQuery)
http://code.google.com/p/flot/
jqchart (jQuery)
http://www.reach1to1.com/sandbox/jquery/jqchart/
Reviewed from smashingmagazine:
http://www.smashingmagazine.com/2007/10/18/charts-and-graphs-modern-solu...
Sparkline
http://sparkline.org/
GDChart
http://pecl.php.net/package/GDChart
Image_Graph, library
http://pear.php.net/package/Image_Graph
http://pear.veggerby.dk/samples/
Libchart, library
http://naku.dohcrew.com/libchart/pages/introduction/
Open Flash Chart, Flash
http://teethgrinder.co.uk/open-flash-chart/
PlotKit, JS/MochiKit (BSD)
http://www.liquidx.net/plotkit/
(uses Google excanvas (Apache license) for more general 2d lib support
http://excanvas.sourceforge.net/
) We've had some good success with this
FusionCharts, Flash (also have a Non-Free version 3)
http://www.fusioncharts.com/free/
http://drupal.org/project/fusioncharts
JPGraph
http://www.aditus.nu/jpgraph/
Non-Free:
Google Charts, image
http://code.google.com/apis/chart/
Drupal Modules:
http://drupal.org/project/charts
http://drupal.org/project/chart
http://drupal.org/project/drawing