Path - If you forgot to add one of the three requirements while uploading a photo on Path for iPhone, the buttons bounce as you tap on Save.
Cloudapp - After pressing the “Login Button” its value changes to “Logging In”, letting you know that something’s happening in the background.
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.
Thunderbird - When receiving a message from a mailing list a “reply list” button replaces the “reply all” button.
houseofbuttons - Playtime of song visualized in button (unknown source)
Google - “I’m feeling lucky” button fades in when you select a result. Hotkey is right keyboard arrow.