The ISV Config offers some interesting options for developers
1) It offers two places where you can add buttons and menus at the application level. The Global Menu Bar and the Application Level Tool Bar.
2) It also offers three areas where you can add menus, buttons and navigation items at the detail level including the Form menu bar, the form toolbar and the form navigation pane.
3) You also have two areas that can be modified in the Entity Grids which include the Entity Grid Action Menu and the Entity Grid Buttons
Make note that if you change the Web Interface you will also want to change the Outlook Interface.