Topic: Login for multiple Projects?

Hello,

is there an easy way to use a User-Model and login-functionality for multiple projects?
For example:
I have a website. Now, i'd like to add a forum. The forum should be a seperate rails-project, but the user shouldn't have to login again, to use it. I want the users to login on the main page and use that login for the forum as well.

Is there an easy way to do that?