* I have never seen such an addictive puzzle so far, http://www.iimi-iris.com/iris/irising/klueLESS/
Kudos to IIMI team and thanks to sweta for pointing it out to all of us at Office. It is looking that my Saturday and Sunday will go into it! Some related discussion can be found here too.
* Another 0ne is: http://www.planarity.net/