GmailerXP is the be-all-end-all utility app for Google's Gmail service. The program allows users to do anything that they can do online, it also combines all the features of the other programs (GML, Gmailto, etc.) The ONLY Gmail app you'll ever need.
More screenshots in Features page.
IMPORTANT NOTE (8.11.04): New 'version' was released that is fixed to work with Gmail's new login procedures. You can download it here.