The second applet that I wanted to rewrite to save it from Java purgatory was a great transverse standing wave applet by C.K. Ng. I used this principally for a data source for students to explore resonance - it's a lot easier to get reliable data with sufficient amplitude variation using an applet for this than a real experiment. In addition to the standing wave amplitude never being overwhelmingly large with a string vibrator, there are hysteresis effects that will drive the kids crazy. I have them collect this data at home, BTW, so the Java issues have meant that, for the last two years, only a handful of kids have successfully been able to use the applet at home and, without anyone for troubleshooting, they quickly give up.
I'll also say that the approach of summing over the normal modes to find the solution for a given f, L, etc. gives a much neater animation than using a finite element/balls and springs model of the string and waiting for the old waves to damp out. It's idealized, but we're really just looking for the steady-state here anyway - this just gets us there faster. It will make the computer work, though!
The most significant difference here is that I haven't created the draggable ruler, opting instead for more prominent gridlines. I always wanted to measure the amplitude anyway, so the horizontal ruler in the applet didn't help much, but using the grid and some arbitrary 'block' unit should be able to serve both purposes.
Let me know if that is an important feature for you, or if there's anything else that you can think of to add or modify to increase the usefulness of this applet! Click through the photo for the applet itself.