Like Windows Vista, Windows 7 sidebar also supports Gadgets which you can download from Windows gadgets library and add it on your Windows 7 sidebar. Now here is an interesting tool Kludgets Engine which lets you to run Mac OS Widgets on Windows 7 & Vista Sidebar.
Kludgets is a widget engine. Dashboard compatible, opensource, webkit based, QT Nokia based & steals the best features from all the widget engines.
Kludgets Engine in Action
(Click on Image to Enlarge)
Download Kludgets Engine to run Mac OS X Widgets on Windows 7 & Vista.