Google will release a version of Google Desktop for the Mac tomorrow that lets users install gadgets -- something previous versions didn't support. At first I was skeptical -- why would Mac users need a sidebar when they already have their dashboard? Well, it turns out there is no sidebar, and gadgets are installed into the dashboard.
Until now, developers were forced to port their gadgets using tools like the Amnesty Generator if they wanted their work to run on a Mac -- but that's a thing of the past.
Gadgets still aren't exactly "universal", but it's getting pretty close -- they work on iGoogle, Google Desktop for Windows, and now Google Desktop for Mac. Not too long ago you were forced to create one version of their gadget to work on iGoogle, and another for the Google Desktop sidebar. Now developers can focus on creating a single version of their application and rest assured it will work everywhere.
Unfortunately, there is still no way to get get gadgets to work on Linux yet, but considering Google Desktop for Mac and Linux were released within three months of each other, it wouldn't be unreasonable to hope for a new version that takes care of that soon too.