Gmail is so fast, it might as well be an application. Why not make it look like one?

Assuming you have your pop-up blocker turned off, this page will open a resizable window with no scrollbar, toolbar, locationbar, menubar, or status line.
The window will open to: "https://gmail.google.com/gmail".
You can click on the example below for the same effect.

Another trick is to make a "bookmarklet" with the following text:
javascript:void(gmail_link=window.open('https://gmail.google.com/gmail', '_blank', 'width=640, height=480, resizable=yes, scrollbars=yes, toolbar=no, location=no, directories=no, status=no, menubar=no, copyhistory=no')); gmail_link.focus(); window.focus(); gmail_link.focus();