Automatically logging in users from another site

I recently added a wordpress blog to my intranet, and everything went well. I am wondering, however, if there is a way to have my users who have already logged into my intranet be automatically authenticated with wordpress. Essentially I need a way to share credentials with wordpress.

If there is a plugin that does this, that is great, but hints toward how/where I would go about implementing this myself would also be just as welcome.

Thank You