Bonobos - The checkout page shows the credit card date you already entered as a credit card.
Geeklist - Shows how long it would take for your password to be cracked in the sign up form.
Gigantt - When you estimate a task using free text you see a live preview of how the system is interpreting it.
/via Assaf Lavie
Facebook - The prompt text in the “Update Status” field constantly changes on refresh.
/via Ahmed El-Agha
Twitter - If you follow a “Tweet” link when not signed in, the Twitter dialog window allows you to sign in & Tweet in one action.
/via Rich Dooley
Apple - searching for colloquial or rumored product names (such as iTouch or iTablet) refers you to the appropriate products.
Kickstarter - The email sign up field proposes related email provider is it thinks you may have mistyped your email.
Simple - While signing up for an account, a Visa debit card is filled with your name as you type it in the form.
/via Tyson Gach
iCloud.com - Removes the spaces from the email input field when you go to the password field.
/via Andrey Usoltsev