Bluff is the most recent graphing library I’ve heard of, so it gets top spot on the list. Prints out some pretty results and is cross browser compliant (as long as you also use ExCanvas for IE). Screenshot below:
The Filament Group released their fgCharting jQuery plugin earlier this year. While graphs in other libraries may look a bit better (Flot?) fgCharting ties your graph to a <table> of data, and editing the values in the table will update the graph in real time. Sexy stuff.
I’ll be honest, the most underwhelming option so far is YUI Charts. Mind you – this is based entirely upon the default theme of the graph. Presumably you could style your graphs to look a bit better (one would certainly hope so!), and their API is far from lacking.