CouchDB could be the starting point that we can build on top of.
We would have to built this ourselves. However, CouchDB has a "_changes" API that can even be turned into "continuous" mode [1]. This is the perfect data source for pub/sub.
hood.ie, Meteor, etc. offer more features, but they are also more cluttered and not as clean as CouchDB.
[1] http://docs.couchdb.org/en/latest/api/database/changes.html