The new Gmail compose window now has a full-screen option! I don't have it yet but I really want that.
When this option is enabled, the compose window is centered in your inbox and expands to fit your screen. The formatting toolbar is also on by default but can of course be turned off. You can click on the expand button in the top right corner to switch to full-screen, or you can set full-screen as the default option, by selecting Default to full-screen in the more options menu in the bottom right. You don't see these options until you get this new feature.
This new full-screen option is rolling out to everyone over the next couple of days. I hope I get it soon ;)