- Mac Dashboard Widgets: the OG of widgets; Google has a few too
- ...copied by MS in Vista: Windows Sidebar Gadgets
- Google Gadgets: desktop or web
- Yahoo Widgets: desktop, within the yahoo RT
- Opera has its own which work on the desktop, mobile phone, or Tv (have to see this one!)
Then there are the 'social sites' plug-in widgets (eg for Facebook: Washington Post's collection of widgets). Also, offerings such as this one that provide the backdrop for widgets to work. Or, for a really useful collection of widgets, look here.
A cool company that authors widgets.