According to its developers, MetroMail surpasses the native Windows Phone 8 mail app by supporting archiving, starred emails and labels; inbox tabs; server-side search functionality and support for advanced Gmail search operators; possibility to concatenate multiple accounts under one unread counter for use with a single live-tile or the lock-screen; and more.
For the securely-minded, MetroMail only connects directly to Gmail servers and never stores your password. It's like logging in to Gmail through any web browser.
Download: Windows Phone 8
MetroMail for Windows Phone 8 screenshots