Cross Virus

A very simple concept but it is very thought provoking, and surprisingly, challenging. You can't "lose" the levels(from what I can tell) but the star ratings give you a goal to work towards. Great time waster.

This fun, simple game betrays it's insidious nature. You'll breeze through the levels, but your measly one star ratings will eat at you. You'll keep trying - and failing - to get three stars on each level. This is one of the best, simplest, and most challenging puzzle games I've ever played.