OmniMail for GMail is an extension for Chrome that makes the omnibox even more ubiquitous by integrating your Gmail contact list to it. With OmniMail extension installed you can quickly compose new emails directly from your browser’s address bar!
To compose a new email, simply type “mail” followed by a space and the recipient's email address in Chrome’s address bar. On pressing Enter, the page will redirect you to Gmail’s compose window with the recipient's email address auto-filled in the “To” form. You can enter multiple email addresses each separated by a space.
You have to grant OmniMail permission to access your Google account. This lets the extension retrieve your contact list so that when you type the first few letters of any email address, the address bar’s URL drop down list offers you suggestions based on names and IDs already present in your contact list.
And you don’t have to remain logged into Gmail to search contacts from the address bar, although you do have to if you want to send mails.