Since I'm actually in the process of building a site that helps you find co-founders, I've just set up a Facebook page that you can "like" to get an announcement when the site is up:

http://www.facebook.com/pages/Find-a-co-founder/155551604502...

(I'm using a Facebook page for this because I haven't decided on a domain name yet, and if I were to ask people to sign up for an announcement email list, some people would be hesitant to provide their email address, I guess).