"Playdar was really a headless, UI-less technology that could be leveraged by online sites and services to let users bring their own content to other people’s contexts.”

Why was Playdar not successful as standalone technology? It would seem applicable to a broad set of use cases for content-addressable data stores.

Calibre (also open-source, also Qt-based) has metadata plugins for various online ebook services. There's a growing number of browser extensions which overlay user data onto websites, many of these extensions would benefit from a widely-used content resolver that works in cross-domain (home, work, mobile1, mobile2, site1, site2) contexts.