OpenMicroBlogging is a protocol that lets users of one microblogging service subscribe to notices by users of another service. The protocol, based on OAuth, is open and free, and doesn't depend on any central authority to maintain the federated microblogs.
The Laconica software that runs SPNbabble supports OpenMicroBlogging 0.1. Anyone can make a new installation of Laconica on their own servers, and users of that new installation can subscribe to notices from SPNbabble.
If you have an account on a remote site that supports OpenMicroBlogging, and you want to subscribe to the notices of a user on this site, click on the "Subscribe" link under their avatar on their profile page. This should take you to the remote subscription page. Make sure that you've got the right nickname registered, and enter your profile URL on the other microblogging service.
You'll be taken to your microblogging service, where you'll be asked to confirm the subscription. When you confirm, your service will receive new notifications from the user on SPNbabble, and your service will forward them to you (using IM, SMS, the Web, or whatever else).