Gadgets are "mini" applications that live within the Vista Sidebar (in the future available for Windows XP as well) and on live.com, respectively for my Swiss compatriots on live.ch.
If you want to develop a gadget, on Microsoft Gadgets you will find this information:
· Users and developers in US (English): As of August 10, 2006, the US (English) version of Live.com no longer directs users to this site and instead redirects to the newly launched Windows Live Gallery. Please submit your gadget to the Windows Live Gallery site today so Live.com users can continue to add your gadget to their homepage. For more information on the transition to Windows Live Gallery including details on a special offer, click here.
· Users and developers outside of US (English): You can continue to use both Microsoft Gadgets and Windows Live Gallery to add items to your Live.com page. Check out Windows Live Gallery (currently US, UK, and AU English only) for gadgets and other cool things to personalize your Windows Live and PC experiences.