- Yahoo Widgets (formerly Konfabulator)
- Microsoft Gadgets
- Apple MacOS X Dashboard, Download Dashboard Widgets
- Google Sidebar Plugins
- Google Homepage Modules
- Opera Widgets. Although these depend on libraries from the Opera web browser, they run outside the browser.
Microformats - Widget Examples has some useful info.
- Canton Public Library provides widgets for Windows Sidebar, Mac OSX Dashboard, iGoogle, and NetVibes