I don't know anything that does precisely that -- but I know both
Opera's M2 and
Lektora (a firefox/IE plugin) have the ability to have persistent searches -- you set up a search and it effectively acts as a filter, of sorts -- a virtual folder collecting all relevant stories. While you're still downloading everything, you don't have to go searching manually for the items you're after. This is a feature I, personally, take great advantage of.