Modifying WordPress Login

Hello,

I’m trying to integrate together three login systems, one of which being WordPress, so, I want to modify WordPress’ login.

I know I can dive into wp-login.php and modify some core stuff to accomplish what I want. However, I’m wondering if there is a plugin method I could use to do this.

Basically, I’ll authenticate the user with one of the other systems, then figure out the user’s WordPress ID, then perform the wordpress login. So, I need to basically bypass the normal login authentication.

Any ideas?

Thanks.