The Kludget Engine is a tool that allows you to run MacOS Dashboard Widgets, Opera Widgets and other gadgets on your Windows desktop.
The program only comes with a clock and a calendar gadget, but in a few clicks you can be downloading others: simple games, translation tools, weather forecasters and more.
And of course you can download and run MacOS Dashboard Widgets, too, although support for these is variable. Any that require user input, for instance, are prone to fail in various odd ways.
For the most part, the widgets are easy to configure and control. The clock and calendar appear when you first run the Kludget Engine, for instance, and right-clicking them will display options to tweak their settings, create copies of the gadget, view the underlying gadget code (handy for web developers), and more.
These are all powered by the Kludgets engine, which has a permanent icon in your system tray. It only consumes a fairly minimal 14MB of RAM (private working set), though, so should be acceptable on just about any system.