Posterous lets you sign in with Facebook or Twitter to comment.
If you sign in with Twitter and check you want to post to Facebook, it pops up the Facebook oAuth window after you hit comment while simultaneously posting your comment to the blog via an ajax call. They also have a very nice tweet composition window.
If you send a chat message on Facebook and the recipient is offline, you can turn it into a regular FB message with one click
Tweetmeme explains why it wants access to your Twitter before showing you Twitter’s oAuth page
From formspring.me’s inbox sidebar
Tumblr allows anonymous Formsping.me-type questions, but warns you first
DonorsChoose.org has this excellent tabbed interface to help you share word of your donation (and encourage others to join you) after the credit card screen
Tumblr’s processing video hold message
Meetup.com has a nice notification of new features that feels like an inbox
Formspring.me advertises Facebook Connect as an alternative to a password, but still gets unique info like email and username
Screenr makes you check the box not to tweet: If you check the box then the “Tweet it” button changes to “Post it”
