Drag'n'drop buttons for Opera: "Note: this page is updated for Opera 8, don't expect all buttons to work in previous releases.
Since Opera 7.10 it is possible to add new buttons to Opera with drag'n'drop. You can share your custom buttons as links on a web page, and visitors can drag the link to create a button on a toolbar. On clicking the link (instead of dragging it) a button will be placed on the 'My buttons' panel of the 'Appearance' dialog."