Oh, how I love free things. Especially free pretty things. And I'm the kind of person who things Flash charts and graphs are pretty, so I present amCharts and PHP/SWF Charts. amCharts is a relative newcomer that creates attractive Flash pie and donut charts based on data you put in a text file in either XML or CSV formats (which makes exporting from Excel easy). It's very configurable, allowing you to specify colors, border, tilt (for a 3D effect), labels, fonts, backgrounds, and so on. amCharts' creators promise bar, column, and line charts this spring.
PHP/SWF Charts is sort of amCharts' big brother. It's been around awhile longer and features more than a dozen different chart types and nearly unli mited configurability. As the name implies, PHP/SWF Charts uses PHP to load data from dynamic sources, but it's not required--there's XML/SWF Charts that will load data from a regular XML file. With its advanced features comes more complexity, of course, and the learning curve is probably greater, but if you need, say, a stacked 3D column chart, it's the way to go.
As I mentioned, both charting tools are free, but not totally free. The free version of amCharts will put a small link to amCharts.com in the corner of your charts, and PHP/SWF Charts will direct users to its web site if they click on it. For a single-site license amCharts will cost you 45 Euros (about US$60) and PHP/SWF charts will cost you US$45. "Enterprise" licenses for unlimited sites cost 245 Euros (~US$322) or $550 respectively.