Twitter - Twitter displays links without the http:// appended. However, when you copy that link, the http:// is appended for a split second.
/via Kevin Ebaugh
Twitter - When moving the mouse over a tweet, the name fades out to show the performable actions.
/via Benjamin Trinczek
Flickr - Error message when you exceed the 140 characters limit when posting a photo on Twitter.
Twitter - Tracks accounts that are viewed by unregistered and logged out users. Then prompts them to sign up and follow those accounts automatically.
/via Jon Wheatly
Twitter - In iOS 5, attachments are aligned so that the scrollbar just fits between the view border.
Feather iOS App - The Twitter bird turns slowly green as you type and goes red when you get to the 140 character limit.
/via Anna PS
Twitter for iPad - Swiping your timeline to the right, removes the content layer. It’s visualized by a little animation behind the timeline.
Twitter for mac - After typing the first few letters of a word, pressing the ESC key will show a list of words that starts with those letters.
Edit: This is a native feature of OSX and works in some other apps as well.