So when Path launched version 2 of their iPhone app a couple of days ago I though "meh". The app isn't even on my iPhone anymore as when it first launched and I checked it out it was YAIPSA (Yet Another iPhone Photo Sharing App). I heard a couple of people mention that it had a very and good UI, so I grudgingly re-installed and logged into my account.
I love it. Love it. I'm not a huge UI guy but even a goon like me can recognize how good it is.
Basically path is like facebook, but private by default, a limit on friends (50) to keep it to your "real" friends (not your facebook friends) and lets you create a news feed (sorry, "path") where you can share things about your life, places, people, thoughts, music, and images. I'm not huge about sharing everything in my life (well, I'd like to think I'm not), but the private by default and limited friend list means that (in theory) you're sharing things that you care about with the people you actually care about.
Again the UI is gorgeous, and all sharing is distilled down to a single button, which works really well. For a extended look and interview with the founders, check out the prolific Robert Scoble's interview below, which includes a bunch of neat tidbits about the design and how they got to where they are now.