Filter-y thing?
You do know you can click on Unread Posts on any category on the main page and it'll only show you the unread posts for that category, right? It's a new feature of SMF 2~
Yeah, but that's different.

*is very particular; stamps foot petulantly* That only displays the list of
threads with unread posts in them, so I have to click on them each in turn. If it displayed the
posts, it would be closer to what I'm saying.

Basically, I think Recent Posts is the most convenient way to keep up with the forum, but it starts to become less convenient as the Recent Posts stream gets clogged up with Tech Support posts and things that while maybe not bad or even uninteresting are from areas of the forum that the user has no interest in or has no interest in at the moment.
Hrmm... did a quick experiment... from the html, it looks like the posts get grouped by topic and topics get grouped by board, but the actual html path to a given post doesn't include the board info... That might explain why this tool doesn't exist... *shrugs*