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.
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.
- Opera Widget Howto*
- Dashboard Widget Howto**
- Yahoo Widget Howto**
*Original links dead, but copies found at the Internet Archive.