FluidDB is a centralized but scalable datastore. In it, the meaning of a given object is derived from the tags placed upon it-- and any user can place namespaced tags on any object. The idea is that ad-hoc standards for object IDs and tag names will emerge through use. I think this is a pretty novel shared-data concept and have had a couple ideas for it, but I'm sure news.yc will have additional novel applications. (I am a fan of FluidDB and its creator, but am otherwise unaffiliated.)
What interesting apps can you imagine with this shared-data system?
You can get a high-level idea of it here: http://abouttag.blogspot.com/2009/08/guardian-1000-novels-everyone-must-read.html
You can get into the details here: http://doc.fluidinfo.com/fluidDB/
That's just a first app. We'll open source it. I can think of many more, as you might hope :-)
Terry Jones (disclaimer: I work for Fluidinfo, makers of FluidDB)