With Gmail you can use any addressbook program which supports drag'n'drop. The best choice is the 'GnomeCard' address book which comes in the gnome-pim package.

You can drag'n'drop entries from GnomeCard into the gmail compose window. You must drag it into the to, cc, or bcc fields.

There is a menu item and a tool bar icon in the compose window to run your address book program. The default is GnomeCard, but this can be configured in the preferences.