This applet lets you play over games and positions (as specified in html parameter lists), or take control yourself and try out variations ("analyze" mode).
You can also set up your own positions using the piece tray.
And in "virtual board" mode, you can move the pieces anywhere.

Illustrative Game: Hikaru Nakamaru - Jay Bonin   Comments

Illustrative Position: King and Queen vs. King mating technique - taking the "Knight's opposition"   Comments

Some interesting chess positions

and I own this experimental lecturing website.