If you use Gmail, many Google services have access to your contacts list and you can easily add addresses using autocomplete or from a contact picker. But what happens when you need to enter an email address on a site that's not from Google? Many news sites have options to send a link to your friends and sometimes this is more convenient than loading Gmail and pasting a link to the article.
Google Contacts Autocomplete is a Greasemonkey script that brings your Gmail contacts everywhere you go on the web. When you start to type the name of your friend or his email address, the script shows a list of suggestions.
For now, the script only provides suggestions for a single email address, but I'm sure this could be easily fixed. Unlike Gmail, the script sorts the suggestions alphabetically, not by affinity.