First, if you want to make multi-session but temp data, you should probably look into the ASP.NET user profile.
If you want to persist logins across sessions, look at the bits of FormsAuthentication that deal with remembering the user.
If you need to keep sessions alive indefinitely without setting the timeout forever (therefor triggering murder by the server admin in some cases), a neat trick is to setup an Ajax "heartbeat" to ping back to the server while the browser is open and effectively do a "keep this session alive" trick.