The Nebraska Astronomy Applet Project site provides high quality interactive simulators dealing with important astronomical and astrophysical phenomena. They plan to have 19 modules, with the current ones covering solar system models, the (apparently) rotating sky, motions of the sun, planetary motion, lunar phases, blackbody curves, Hertzsprung-Russell Diagram (shown here), eclipsing binary stars, extrasolar planet detection, and variable star photometry (including a simulated blink comparator). They also have background materials to help you understand and use the simulators, and instructor materials to help teachers.
These things are so cool! They can really help you to visualize astronomical phenomena better than static diagrams. Thanks to Jacek Kupras of U Kuprasów for the tip! This is a great blog that covers many of the same topics I do, only in Polish. I occasionally use Google Translate to read some of the posts there when the pictures look interesting. Years ago I studied (and mostly forgot) Russian, so some of the Polish words are familiar when I sound them out (common Slavic roots), but not enough to really understand without Google's help.