Cookie Clicker
http://orteil.dashnet.org/cookieclicker/
Once you're bored of that, there are... a lot more[3]. CivClicker is particularly enjoyable[4].
[1] http://candies.aniwey.net/
[2] http://candybox.wikia.com/wiki/Computer
function a() { document.getElementById('bigCookie').click(); window.setTimeout(a, 100); } a()
EDIT: better: Game.cookies = Game.cookies*2