Everyone is going widget crazy! Microsoft (who calls them gadgets, not widgets) has them on the Start.com and Windows Live sites, on the Windows Sidebar, and with the help of laptop vendors, on special secondary laptop screens branded Windows SideShow. Yahoo! provides its Yahoo! Widget Engine (formerly Konfabulator) at no charge for both XP and OS X. Apple provides native support for gadgets on its Dashboard, and finally, Stardock supports them as stand-alone programs or as part of Object Desktop (and provides some of the best developer tools). Of course, these are just the ones that I have personally had exposure to; there are also many Open Source projects with similar functionality.
As usual, none of these systems are designed to work together. The Yahoo! Widget Engine and OS X’s Dashboard seem to be the most compatible, but this appears to be due to their shared genealogy, not an actual attempt at ongoing compatibility. Currently I have been using the Yahoo! Widget Engine since I need to support both Windows and OS X users. So far I have found XMLRPC based widgets used to monitor news feeds to be the most useful. There are a lot of example feed readers out there, but the value for us really became apparent when we started creating custom feeds based on business data. Today I created a new feed for one of our development groups so the QA guys can watch for new and updated bug reports without sitting in the bug tracking application all day.