My users have started using Fuelly for tracking MPG on their cars. So I've created a custom profile field where they can add their Fuelly profile name.
I've then created a member list based of this information in a page with a PHP callback.
Nice and simple, but very effective. I've taken the code from the notable members where it's generating the Post Count, Like Count etc, and turned that into the Fuelly username with a link to their account on there.
It's great working with XF!