Updated: I just got an invitation to the service through this blog post (check the comments).
When Crazy Egg starts accepting signups for its free service there will be a stampede. Pete Cashmore of Mashable is gushing: “Wow! Just wow. Crazy Egg will absolutely blow you away.”

Crazy Egg is a website tracking script that will tell you where your users click on your site. It will then generate a heatmap overlay on your site to show you where your users are clicking.

Cashmore says in the comments of his post that you just use “a tiny bit of script” for the service. Updated: you just put in a two-line javascript to start tracking.
I’ve signed up for the waiting list and there’s this nagging thought to keep on signing up and pester the service into giving me an account. Here’s a screenshot I took from the Crazy Egg site: