OS X: Odds are, your Mac's Dashboard has fallen by the wayside in recent years (even though it can be really useful), but widgets can still be really handy for certain tasks. WidgetRunner frees them from their prison in the Dashboard, and lets you run them alongside your desktop apps.
There's an old trick to do this with a terminal command, but it doesn't seem to work for most Mountain Lion users. WidgetRunner works on modern Macs, doesn't require a terminal command, and doesn't even saddle you with a complicated interface to set everything up. Just open the app, click the main dropdown menu in your menubar, and hit "New Widget". Your custom-installed widgets can be found in /Users/username/Library/Widgets, and the default widgets will be in /Library/Widgets. Just click the ones you want, and they'll populate on your desktop where you can drag them wherever you wish.
By default, widgets will act like regular windows, and disappear behind any other active applications. But if you want one to float on the top of the screen, just right click it and change the widget position to "Top". You can also set widgets to "Desktop" mode, which effectively turns them into part of your wallpaper. You won't be able to interact with it at all unless you first click the WidgetRunner icon in your dock, but it's great for passive information widgets like weather or stocks. The relevance of most widgets has been diminished by more powerful web apps over the years, but if there are any that hold a special place in your heart, this is a great solution to keep them close at hand.