Netvibes, my favorite personalized homepage, is a site that thinks big. There are so many platforms for widgets/gadgets/modules, and each platform uses its own format, so the most popular personalized homepages got the most widgets.
Netvibes has just launched a universal widget API, that should let you develop widgets for Netvibes, Google Personalized Homepage, Windows Vista, Apple Dashboard, Opera etc.
"The Universal Widget API (UWA for short) is the name of the 1.0 release of the Netvibes API. With it come major changes and possibilities. While previous version of the API only let you build modules for Netvibes, UWA makes it possible for you to see your widget be used not only on Netvibes or on personal websites, but also on many other platforms, both online or on the desktop. (...) Thanks to its open-source JavaScript runtime, the Netvibes UWA can be easily ported to other platforms. As a developer, you can leverage your existing code on a large number of platforms. As a user, you can use all your favorite UWA widget on your favorite platform - it doesn't have to be on Netvibes only."
There are some sample widgets here and you'll notice they keep their Netvibes look on other platforms, which is not a good thing. But Netvibes gained a lot of points by inviting developers to build gadgets using their API: their mission is excellent and the number of widgets built for Netvibes will increase.