I'd like to set this up as a post where all known JavaScript client development platforms could be listed by company/organization/ecosystem.
* anything that is HTML/JavaScript-based.
Microsoft
Windows Gadgets*
HTA - HTML Applications*
Adobe
Adobe AIR* Can be done with only HTML/JS but can also can use Flex/Flash
Adobe Director
Adobe PDF
Yahoo
Konfabulator
Google
Google Desktop Gadgets - SDK link on the left
Mozilla/Firefox
Greasemonkey
XUL Runner
Appcelerator Titanium
What did I miss? Should I also list JScript.NET, managed JScript, Rhino?
note: I've set this up as a community wiki.