Windows Vista Sidebar gadgets are little software programs that run inside of the Windows Vista Sidebar.
A Windows Vista Sidebar gadget might be a clock, a weather radar map, a Facebook wall interface... just about anything.
Some gadgets, like these Windows 7 gadgets (yes, they work in Windows Vista too), can even act as little system monitoring tools, making sure your computer isn't overheating or running out of battery power.
Here's how to download an install a gadget to your Windows Vista Sidebar:
Download the gadget.
Microsoft used to catalog and host Windows Vista gadgets but they no longer do. Today, you'll find most gadgets for Windows on software download sites and on the websites of gadget developers.
Run/Open the downloaded GADGET file.
Windows Vista Sidebar gadgets are contained in a single file with the GADGET extension.
Click Install if you see a "Publisher could not be verified" security warning.
The majority of Windows Vista Sidebar gadgets are developed outside of Microsoft and not all of them program their gadgets under the strict requirements set forth by the company. However, that doesn't mean there's anything wrong or dangerous with the gadget.
Setup the gadget.
This step is completely dependent upon the gadget you happen to install. For example, if you install a Facebook gadget, the gadget will need your Facebook credentials. If you installed a battery level monitor, you may want to adjust the size or opacity of the gadget window.
Not a Windows Vista user? See How Do I Install a Windows Gadget? for instructions specific to your version of Windows.
Sidebar gadgets are not supported in Windows operating systems before Windows Vista but plenty of third-party gadget programs do exist.
If you remove a gadget from the Windows Vista Sidebar, the gadget is still available on your computer, it's just not installed.
To reinstall a previously used gadget back to the Windows Vista Sidebar, just right-click anywhere in the Sidebar and choose Add Gadgets... from the menu that appears.