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.
There's also Titanium.
http://www.appcelerator.com/
You can use JavaScript, Python, or Ruby for desktop apps. I believe that mobile apps are JavaScript (no Python or Ruby).
In this context the Universal Widget API might be interesting - it's designed to be a framework to write widgets for different platforms (desktop or online):
http://dev.netvibes.com/
Widgets written with the UWA work on iGoogle, Windows Vista, Apple Dashboard, Live.com, iPhone, and Opera.
Windows Mobile 6.5 (MS Smart Phone OS) will support HTML/JavaScript Widgets similar to Vista/Windows 7 Widgets:
Windows Mobile Developer
Looks pretty interesting.
