You know, it's too bad that one of the first most public cross-site mashups is as evil as the Facebook Beacon. I've been thinking for awhile now about what could be done in a JS-based widget using cookies and identity.
The trick is that the site including the script can supply parameters describing the host page, while the cookies sent along with the request to fetch the script can describe the user to the widget provider. Put the pieces together in a browser by dynamically generating a tailored widget, and some interesting things could happen.
Rather than collecting marketing telemetry, I'm primarily concerned with figuring out how to do something cool for a user. Given data about the widget's current guest environment and the person now viewing it, we could come up with personalized results and recommendations — maybe even enable some tailored in-context functionality.
For example, what if blogs could include a little sidebar widget that displays and allows you to manage what notes and tags you've saved for that particular page? Or if you could see related links for the current page, based on your collection of tags and interests? Or if you could see what your contacts have said about that page?
There are still privacy concerns to examine here, and anything like this should be opt-in only — but I think something very interesting could happen in this intersection of context and identity.
I just hope that this beacon fiasco doesn't prompt the web immune system to respond by killing this scenario entirely.