I've recently stumbled upon Enigma, which is an open-source remake of an old game called Oxyd, which I used to play back in middle school on Mac OS 7. It's an incredibly absorbing mix of puzzle-solving, coordination, and simple but elegant graphics. I had forgotten all about it, not having played it in more than eight years, but it took me about 30 seconds to become re-addicted. It's been ported to all kinds of platforms, so you have no excuse not to play it.