I was wondering how a site like Stack Exchange manages to have a unified login system for all of their sites. Is the login/user management system kept separate from the sites?
All sites use OpenID. Accounts are linked through the network on the server side if they use the same OpenID.
Regarding the network login (when you open an SE site while logged into another SE site you get automatically logged in), that is done using StackAuth.com with AJAX (or iframing, not sure which). When you log in, Stack Auth sets some cookies as well. When you visit a site, a request is sent to Stack Auth, which realizes that you are logged in on to the network, and you are automatically logged in.