The number of Gmail tools available is rapidly growing and this is a good sign of interest for the new Google mail service.
The last nice tool I've discovered is the Gmail Image Viewer.
If you receive a mail with multiple images as attachments, with Gmail you've to click each one to open and see it. If you want to see all the images at once, this is the right tool. Gmail Image Viewer is a small browser add-on that adds a right-click menu item that will automatically inline image attachments in your Gmail messages in Internet Explorer. Nice... 
I've not tryed the tool yet but after a quick review I've a big lack (for me obviously!): the tool is a Javascript code that is installed into the installation directory and a registry entry (HKEY_CURRENT_USER\Software\Microsoft\Internet Explorer\MenuExt\Gmail Image Viewer) that adds the relevant menu item to Explorer. Will it work with Firefox????