[Yahoo] – has extensive API documentation and tools for creating Yahoo Widgets
[Widget Converter] – allows fast conversion between flat widget files and a directory structure to bundle and analyze widgets
[Konfabulator] – provides online manual documentation on Widgets
[Widget Creation Tutorial] – is the first thing you should read before making your first Widget
[UniView] – is a handy tool to help encode text properly
[Forums] – to chat with fellow widget creators
[Apple] – provides widget design guidelines
[Microsoft] – provides design guidelines when creating Widgets
[Yahoo] – has an article about using Flash in Widgets