I stumbled upon this a few years ago and found it charmingly clean, maybe too clean (I recall my parents being confused about the UI). It provides an excellent service all while not saying much about itself.
It hasn't been mentioned much on HN, maybe because there isn't much to say, but I love the delivery.
As someone who is currently working on a browser game myself, PlayOK feels like optimization nirvana. It's so refreshing to see such a minimalist approach in a world where no one cares about bloat any more.