Internet Explorer uses a proprietary add-on system.
Having standardized extension coding could lead to less work for developers, who could make one single extension for a plethora of browsers instead of needing to re-code every time.
Says Opera:
Extensions allow you to easily add new functionality to your Opera browser. Developers can create extensions using familiar open standards: HTML5, CSS, and JavaScript, along with supported APIs. To learn how to create extensions, have a look at our introduction to Opera extensions.
A selection of add-ons can be chosen from our new extensions catalog. With Opera 11 alpha running, you can install extensions by just clicking on them. A small install window will appear. Click “Install”, and you are done. If you have an extension file stored on your computer, it can also be installed by dragging and dropping it into the Opera window.