The Opera web browser has introduced a Dashboard-like Widget feature in Opera 9 Preview 2. I believe this is the first 3+ platform widget framework out there.
Dashboard is, of course, Mac OS X only. Yahoo! Widgets (formerly Konfabulator) is Windows XP and Mac OS X only. The KDE Desktop (mostly used on Linux and *BSD), has plans to include floating applets in KDE4′s revamped desktop, Plasma. Opera runs on Windows, Mac, Linux, and a number of other platforms.
Opera’s taken a very similar approach to Apple’s. A widget is a bundle of HTML, CSS, and JavaScript. The most obvious difference is the structure of the bundle: Dashboard widgets are in a whatever.wgdt directory that’s been zipped, and Opera widgets are in a zip file that’s been renamed as whatever.wdgt. On the Opera Forums, Non-Troppo determined that they use different XML formats for metadata (name, version, author, etc.), and Rijk suggested that “As long as the Dashboard widgets don’t use Mac-specific API, it should indeed be easy to convert them.”
Yahoo widgets are a completely separate XML format, but it looks like it should be possible to build a widget that works on both Opera and Dashboard, or at least one you can convert easily to two downloads.
I belive there are more than three.
Opera Widgets, Microsoft Gadgets, Apple’s Dashboard, Google’s Widgets and Yahoo! Widget Engine (Konfubulator).