Chess has long been the quintessential training ground for aspiring geniuses, warlords and fun seekers who just enjoy a strategic clash of minds.  My school has a chess club including a life size chess board and regular tournaments and I really encourage the kids to get involved with it to exercise their minds and learn to deal with victory and defeat in a diplomatic manner.  Anyone with a copy of Windows Vista up has a pretty solid version of Chess that they can play online or in single player mode but I came across this link on Gizmo's freeware that has some sites for the hardcore chess player to test their wares.  Check it out here and feel confident letting your kids play Chess on the computer as a viable educational experience.