Service for creating a browser toolbar

Hi,
Has anyone created a service that would allow you to create a customized toolbar that could display information as a user navigates from one web page to the next? I guess this would mean integrating with existing libraries for the browswer toolbars? Not sure how "open" those are.

Notificaitons are another way to persist messages and info across sites, but having a toolbar puts the information closer to the web page.

Thanks!
John