Google+ - Guesses where a user might work based on where their friends work.
/via Artem G.
Confluence - Username is auto-populated as you enter your email address, and stops when @ symbol is typed.
Instructables - Offers a variety of gender options when signing up for an account
deviantArt - Visual prompt to encourage users to fill in more details about their profile when registering.
Google - Google wishes you happy birthday based on the information from your Google+ profile. Clicking the doodle sends you to your profile at Google+.
/via Alexey Gornostaev