I used a couple but ended up writing my own.
If you need something to start off with, try ChartDirector. I built a real-time control on top of it and it worked pretty well. It's not too nice on resources though and on .Net the GC proved to be too much.
ChartDirector is very full featured though so if you need fancy indicators, etc. it will do a lot straight out of the box.
If you use Java, <shudder>, then there is JFreeChart.
For Javascript, there is the HighStock stuff, as someone mentioned, but after playing with it for a while I found it too much trouble to customize and not efficient enough for what I am doing. Besides, building a full-blown GUI framework in JS is a pain even with all the latest advances.
My advice, if you have a week of coding time, roll your own. You'll make it as efficient as you like and learn a ton also.
/Wulfrede