Google has made available, you Gmail contact list across Google services, so you can easily add addresses using autocomplete or from a contact picker. But what if, you need to enter an email address on a site that's not from Google? Google Contacts Autocomplete is a Greasemonkey script that brings your Gmail contacts everywhere you go on the web by creating a autocompleter next to email form boxes with your Google Contacts similar to the list of contacts when using the “to” field on Gmail. You have to be signed into Gmail for this to work. Handy for filling “send to a friend” forms.
Source:→ Google System