I wish there was a central, fully customizable, open source, universal login system that allowed you to login and manage all of your online accounts (maybe there is?)...

I just found RPXNow today after starting to build a Sinatra app to login to Google, Facebook, Twitter, Amazon, OpenID, and EventBrite, and it looks like it might save some time.

But I keep wondering, not being an authentication guru, why couldn't I just have a sleek login page saying "Enter username and password, and check your login service", and then in the background either scrape the login page from say EventBrite and programmatically submit the form with Mechanize, or use an API if there was one? It would be so much cleaner and such a better user experience if they didn't have to go through popups and redirects and they could use any previously existing accounts.

My question is:

• What are the reasons why I shouldn't do something like that?

I don't know much about the serious details of cookies/sessions/security, so if you could be descriptive or point me to some helpful links that would be awesome. Thanks!