With all this talk about different puzzle type computer games, I thought I'd bring up Chromatron. They are highly addictive and difficult. The author gives away the first game and charges a reasonable price for the others. It took me a while, but I got through all 200 boards.
Basically the game involves different lasers and mirrors and you have to light up all of the targets. It gets tricky because you are limited with the mirrors and stuff that you have.
go to http://www.silverspaceship.com/chromatron/
for info on them.
If you guys need any hints on any of the boards, lemme know.
BTW, I'm not affiliated in anyway to the author, just a happy customer.