I'm a bit of an 'indie' game geek, so love it when a gem like Cavestory or World of Goo pops up.
Anyhoo, I played the first version of Dyson ages ago when it was entered into a procedurally generated game making competition and loved it then. It doesn't try to do much, but it knows exactly what it is trying, and I love these kind of games.
http://www.dyson-game.com/
It's come quite a long way now but it still keeps its simple charm, so if you've got 10 minutes or maybe 48 hours to kill, give it a go.
Ok to give this thread a little more body than me just saying I like a game. What other games do you recommend that fit the role of simple yet brilliant?