I have got chronically addicted to playing ATC-SIM.
It is a browser based Air Traffic Control simulator, that isn't the normal graphics of aircraft that you control with mouseclicks. This is a pretend radar screen purely with symbols, you have to control the aircraft by inputting commands to change their heading, designate runways to land on, not crash into each other etc. It takes a bit of getting used to, but as soon as you have memorised the commands, you are away - reading the instructions is essential!
It keeps a track of how many aircraft you successfully take off and land and the length of time you spend in violation of proximity. I would also suggest not trying to start with Chicago airport as it is far too busy! Heathrow is good because of having 2 runways and a good set of fast to type waypoints around it. 'Normal' difficulty is very much harder than 'Easy'!
http://www.atc-sim.com/
