One possibility for this would be Shendal's Perl/Tk Newest Nodes Client. It shows you a threaded interface to Newest Nodes, and has a persistent cache of which nodes you have already seen, so they remain marked as seen even across sessions, and even before you click on "I've read all these".

Right now it re-reads the Newest Nodes page every time it starts, so it doesn't really do what you want. But it shouldn't be too difficult to add that feature - a flag that tells it not to re-read things from Newest Nodes, but to just reuse its cache. It might be more complicated than that, I haven't looked at the code in a while, and unfortunately don't have time to do it now.

Of course, you could also just leave the client running all the time, which gives you a frozen image of the Newest Nodes, until you press "refresh".

--ZZamboni