A few others that have not been mentioned:
For mini pies, lines and bars, Peity is brilliant, simple, tiny, fast, uses really elegant markup.
I'm not sure of it's relationship with Flot (given its name), but Flotr2 is pretty good, certainly does better pies than Flot.
Bluff produces nice-looking line graphs, but I had a bit of trouble with its pies.
Not what I was after, but another commercial product (much like Highcharts) is TeeChart.