iOS - autocomplete text has the same typeface and style as the underlying text field
/via Idan Gazit
Steam - After purchasing a game from the webstore or mobile app, it allows you to remotely install the game to the computer that you are logged into Steam with.
Foursquare iOS app - if you drag your uploaded photo to the left, you see a hidden mascot holding a camera.
Facebook Camera - After clicking the install now button, a pop-up message offers to text you a direct link to the app, with your phone number pre-filled in.
/via Taylor McKnight
Twitter - On the mobile version, the error message lets you know that typing on a phone stinks.
Eventbrite - When using Mobile Ticket option, it automatically increases the screen brightness temporarily for easier scanning of the QR code.
/via Phil Metcalfe
iOS 5 - When the phone is locked, you can “slide to view” the notifications themselves, taking you directly to the app.
Google Url Shortener The feedback of how to copy the link changes, depending on the device and OS