Changing Google Apps. Interface

We want to add a Link/Button or Links/Buttons in Google Apps. to export the data from Google Apps. to any other application. We want to export GMail Contacts Data, Calender Data, Documents, etc. from Google Apps to other applications. For example. We are looking to make some ammendment in GMail user interface. We want to add some link/button to export GMail Contacts Data to other applications, changes some link behaviour, also we want to change the content of tooltip or baloon which is shown when mouse hovers over Contact links or Email address. Does anyone have any experience in doing this? Can you provide a sample code?