I'm guessing something like this already exists, but I made a chrome extension to hide posts from the first page of the News/New/Ask/Show tabs that were there the last time the tab was loaded.
If I can't break my habit of reloading HN every 15 minutes, at least I can eliminate having to scan stories I've already seen.
https://github.com/rmb177/hn_saw_it_last_time
