Many of the harder puzzles are both beautiful and challenging.
http://armorgames.com/play/2205/light-bot
edit: Ah, I didn't notice there was a javascript version too. Oh well.
funny that they copy a game and make it more boring then the original, in light bot you get to program robots in 2.5D and not arrows.
light-bot is better :edit: too late
Many of the harder puzzles are both beautiful and challenging.