Kickstarter - After filling out a survey (for selecting pledge awards, providing address, etc.), the site gives you 10 minutes to edit your responses. This works even if you exit the site and return.
/via Paul Robertson
GetSatisfaction - Gives instant feedback when using too many capitals in your title.
Google+ - Sending feedback in Google+ gives you the option to highlight the problem and blackout personal stuff.
/via Robert Winters
YouTube - If you are not logged in, the “respond to this video” textarea asks you to Sign in or Sign up on mouse-over.
/Via Marian Buhnici
YCN - Online competition submission form allows you to copy entry details from the previous entrant that you’ve entered.
Mailchimp - Makes you write the word DELETE if you want to delete a mailing list.
MobileMe - When filling out the email form, the text size will decrease to fit inside of it.