While writing Facebook applications, it is useful to create test users in the developer.facebook.com “Roles” tab for your app, and login as those users.
But, Facebook test users are a little tricky to login, since they don’t really show you the e-mail or password of these users. You can get a unique “already logged in” url by asking Facebook a question, and here is a short script that does just that.
I use it in Chrome Incognito windows to test various features with these throw-away users, and it works great. Enjoy.