What is the Sidebar?

The Sidebar is a feature first introduced in Windows Vista which allowed you to personalize your desktop with compact size XML-based applications (referred to as gadgets in Windows Vista and Windows 7) that would allow for quicker viewing and access of information on your computer. It used core design philosophies of Windows Aero to deliver contextual information when requested, i.e., A media gadget which shows audio controls only upon being hovered over. Unlike many features and applications that made their way into Windows Vista from the development period of pre-reset Longhorn, the Sidebar was conceived before development of Longhorn even began in the early 2000's. 

Originally conceived as Windows Sideshow, it began development in the summer of 2000 and was being used internally at Microsoft. It was later demoed at the Professional Developers Conference 2001 keynote by Rick Rashid, running on Windows XP and seemingly Windows 2000.  In the demonstration, Rashid went over the practical uses for the feature, such as being able to quickly see which colleagues in a list are online or offline at any given time through dynamically updating information. A single build of Sideshow is publicly available for download as it was leaked by the WiNBETA team. Sideshow refers to its applications as Tickets. Many of these Tickets are nonfunctional. If you wish to use the Tickets feature, you need to download and run a patch. Both the application and patch can be downloaded here.

Rick Rashid demoing the practical use of Sideshow (PDC 2001, at 57:51)

The Sidebar was included in Longhorn builds as early as the first build to publicly leak, build 3683, where it functions very similarly to Sideshow. In Longhorn, applications (referred to as Tiles in LH) were developed through the use of both .NET and Avalon. The Sidebar is also a part of Windows Explorer and not its own separate process, like it is in post-reset builds, meaning it is prone to causing memory leaks, hangs or crashes. In build 3683, you can customize various aspects of the Sidebar, including its location on the desktop, its size, its transparency, what Tiles you would like to use on it, and so on.  You can even combine the Sidebar with the Taskbar allowing you to have the functionality of both GUI elements in the same location. The Sidebar persisted through Longhorn's development, although it went through some design changes and had new Tiles being developed for it. 

As Longhorn underwent its development reset in August of 2004, many features which were slated for the project were ultimately shelved. The Sidebar however, had not been shelved and made its way into Windows Vista nearly half a decade later. To prevent hangs or crashes in Windows Explorer, the Sidebar now runs in its own separate process as opposed to being a part of Explorer. You can place it on either side of the Desktop like in pre-reset builds, and Tiles have been renamed to Gadgets which serve the same intended functionality. Developers could create their own Gadgets and share them, and you could also download additional Gadgets from Microsoft's website.