Wikipedia page on the vCard format, which includes a list of commonly-used private extensions (X-).
I was talking to Blake the other day about Plaxo, and about how the need it tried to fill (keeping everyone’s contact details up to date) was a valid one, but that it really didn’t live up to that goal. That got me thinking about how a big hole in the distribution of contact details was that you couldn’t “subscribe” to a vCard (contact details) in the same way that you can to an iCal (event/calendar details). Let’s fix that.
I’m imagining an online service (perhaps even just a WordPress plugin?) where you can set up URLs that point to either vCards that are online, or web pages that contain hCards. The system would then periodically (daily?) parse those URLs and load the details into a local cache/database.
The contents of the local cache would be exposed via an LDAP directory, allowing you to connect products such as the Apple Address Book to that directory. Those details would automatically be up-to-date, based on the last time their source URLs were parsed.
This would effectively eliminiate part of the need for services like Plaxo, and would give each person control over their contact information. Ideally the requests could be authenticated so that people sharing their contact details could control their distribution. With DiSo on the way, this would be hot.