Not sure if the title is quite right for the question but I can't think of any other way to put it..
Suppose you wanted to create multiple different web apps, but you wanted a user who was logged into one app to be able to go straight to your other app without re-logging in (assuming they have perms to look at the other app as well). If I'm not mistaken, if you're logged into gmail you can go straight to your iGoogle, googleReader, etc without re-logging in (if you set it up right).
How would you approach this? What would you use? Assume the apps already exist and you don't want to change the initial login page for the users.