Geeklist - Shows how long it would take for your password to be cracked in the sign up form.
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
Confluence - Username is auto-populated as you enter your email address, and stops when @ symbol is typed.
Twitter - On the mobile version, the error message lets you know that typing on a phone stinks.
Tumblr - The copy inside the age verification form, when signing up for Tumblr, changes depending on the value.
Path - Shows a smiley on the pressed state of its signup button.