If you have an Xbox, go play Braid. The gameplay and presentation are near-perfect and, as far as I can tell about a third into it, well-deserving of the megascore.
Chronotron on Kongregate (it’s free!). You play a robot who can go back in time. This lets your present and past selves interact with each other and the environment, sometimes several times over. It’s much more explicitly puzzle-y than Braid, and less touchy-feely artistic, but its gameplay is brilliant.