Mark all as read

I'd like to have a button added in forums (possibly everywhere) that would allow me to mark all comments/stories/entries displayed on the page as read (i.e. not new).

Use case: I'm relatively new to drupal.org. I'm not necessarily interested in reading all of the discussion from months or years ago, but they all show up as "new" items to me when I'm logged on and I view the site. What I'm really interested in (what I want to count as "new") is what's new to the site since I started reading actively. I'll pick a few old threads to read, then I want to mark the rest as "old" and use the (## new) flag in the forum header to see what's really new to me.