I find that sometimes I just want to play a simple game because my mind is not up to complications :P
Below are some rather addictive game links (free of course):
Fishy: http://www.addictinggames.com/fishy.html
Helicopter Game: http://www.helicoptergame.net/
Lemmings: http://www.elizium.nu/scripts/lemmings/
Feel free to add any of your "Boredom Killers"