Tinker

It's free, it has achievements and 160 levels. Visual design is excellent and any system can run it. Very simple, pick up and play puzzle game. Good variety of elements and superb level design given the miniscule size of each arena. There is thought and simple logic and strategy here, but it is very easy to pick up and play since the game paces progression perfectly with its tutorial. My only complaint is about the controls. It's not very fluent due to the nature of the block movements and the level. The camera could be better I suppose but you can change the angles easily. Overall it's a great casual game, fun to play but still quite skillful and challenging. Best of all, it's free and it gives you gamerscore. What more could you want for a quick time-waster?!