The one I've used are JChart2D and JFreeChart. I did a live plotter application over the summer and used JFreeChart for that. The guy who had started the project had used JChart2D but I found that it doesn't have enough options for tweaking the chart look and feel.
JChart2D is supposed to be very fast so if you need to do live plotting have a look at it, although JFreeChart didn't have any problems doing a plot a few times per second.
There also quite a list of charting libraries on java2s.com
