Mac OS X - When searching in the help menu of an app, the menu relevant to the result is pulled down with a blue moving arrow pointing at the menu item.
/via Stephen Litt Belch
Delivery Status app - It detects if you have a tracking number on the clipboard on launch, and it also says from which courier.
/via Patrick Patience
Twitter for Mac - After installing the app, right click on any text in Mac OS X to see a tweet option.
/via Brendan Lee
Offcial Twitter app for iPhone - Dragging down the whitespace of reply screen shows what tweet you are about to send reply.
Reeder app - website makes use of some nice CSS3 transitions.