Picked this up from ReadBurner (hey another social aggregator), it’s fav.or.it which is integrates commenting with rss “river” feeds. It’s still on closed beta right now, but that’s no reason for you not to pre-register and jump right on the bandwagon once it opens up!
Check out the original article and some screenshots on Mashable.
If you’re deciding between this and FriendFeed which I just blogged about, why not go with both and see which you prefer in the long run? I’ll be uniquefrequency on both.