There was something very similar to this for reddit that was very handy and useful. It was at redditsnapshot.sweyla.com but it shut down at some point. I still think there is an opportunity for someone to index reddit's front page and have it hour by hour and maybe a premium version of all the subreddits you subscribe to.
When I saw that sweylas's stuff has disappeared, I asked them in an email if it will be back again. I didn't get an answer so I implemented my own version: http://redditsnapshots.com/ (running since March 2012)
