Change text "Compose Mail", "Contacts" and "Sign out";
Change toolbar background color;
Change labels background color;
Change buttons, textarea and input style;
Change Text decoration menu;
Group contacts button;
Hide Invite and Footer
and other little changes...
Updated (29/05/08) : border style for contacts e cmpose mail links
P.S.
1) Thanks to justtrying for the Gmail - Hides Invites & Chat Boxes
2) Thanks to gnfoster for the Gmail 2.x - change and cleanup header