Using a rather clever browser hack, someone's programmed a playable Defender clone into a favicon. If you don't know what a favicon is, it's that little 16 by 16 pixel graphic that appears in address bars and browser tabs. See that pink and chartreuse "K" up there? That's what I'm talkin' 'bout.
And if you don't know what Defender is, it's a classic sidescrolling arcade shooter released in 1980. Head on over to the Defender of the Favicon site to check it out. Won't take but a minute and fellow web developer types might just get a kick out of it.