Geeklist - Shows how long it would take for your password to be cracked in the sign up form.
Gmail - A cheeky error message is displayed when attempting to use consecutive periods.
/via Simon Souris
Kickstarter - The email sign up field proposes related email provider is it thinks you may have mistyped your email.
/via renaud
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.
/via Brian
Twitter - On the mobile version, the error message lets you know that typing on a phone stinks.
/via @oi_joris
Tumblr - The copy inside the age verification form, when signing up for Tumblr, changes depending on the value.
/via okdan
Path - When signing up as a new member, the iPhone app pulls your contact information from the address book.
/via Mark Oleszczak
Path - Shows a smiley on the pressed state of its signup button.
