It depends on what you want to want to do - this library has a wider scope than others I've seen. The closest I know would be Raphael, which is nice in that it also supports IE (Canvas): http://raphaeljs.com/

If you just want Graphs/Charts, here's a few off the top of my head:

http://www.deensoft.com/lab/protochart/

http://www.liquidx.net/plotkit/

http://code.google.com/p/flotr/

for clarification, raphael's IE support is not from HTML 5's canvas tag, but from SVG's precursor, VML: http://en.wikipedia.org/wiki/Vector_Markup_Language

I'm the author of Grafico, A Raphaël (js based SVG/VML) charting library with a focus on proper charts (per Stephen Few/Tufte) Check it out here: http://grafico.kilianvalkhof.com/ and this page has a lot of examples: http://grafico.kilianvalkhof.com/documentation/index.html

The Javascript InfoVis Toolkit: http://thejit.org/

and gRaphaël: http://g.raphaeljs.com/

spring to mind.

Highcharts has better cross-browser compatibiltiy - http://highcharts.com.

Though it's not F/free, if you care about that:

http://highcharts.com/license

JIT is great http://thejit.org/

Bluff (a port of Ruby's Gruff) does less, but does it very well. http://bluff.jcoglan.com/

I wrote a library that uses canvas: http://github.com/biilly/doodle-js