Massively Multiplayer Open Source Authentication
Besides providing simple, secure, extensible Web single sign on at many colleges and universities, Jasig's Central Authentication Service is also the basis for the current web login to Sony's FreeRealms massively multiplayer online role playing game. With over two million players as of May 2009, this may be the largest userbase deployment of CAS in the world.
Blizzard Entertainment used CAS in the past for Web authentication to the ancillary services (forums, account management) for their immensely popular World of Warcraft massively multiplayer online role playing game. I don't know whether Blizzard continues to use CAS today, but the login form looks CAS-inspired but not-actually-CAS to my eyes.
Personally, I think it's pretty cool to to see CAS used at massive scale, for these massively multiplayer online games. For serious needs like those of financial services firms. And even for really serious needs like access to crisis consulting resources. For practical needs like logging in to a magazine publisher's website and a webmail provider. And at all sorts of places, including the deployments registered on Jasig's website.