The sidebar is a term that is used for a GUI element that displays various forms of information to the side of an application or desktop user interface.
In a number of Widget engines, one is able to install applets (known by various trademarked terms as "widgets", "desklets", and "gadgets") which can reside within or outside the sidebar. Notable examples include:
- Windows Sidebar, made by Microsoft available on Windows Vista only
- Sidebar, included in Google Desktop
- Desktop Sidebar
- Thoosje Vista Sidebar
In specific desktop applications, such as the Opera web browser, OpenOffice.org, SoftMaker Presentations and Windows Explorer, one is able to view various features (that are allowed by the developer(s) of the application) within the sidebar of the application.
Other terms and variations 
Sidebars can also be positioned to the bottom of an application window, such as in Adobe Photoshop.
Mac OS X 
In a number of predominately-Mac OS X-based desktop applications, drawers, which draw out of the application window rather than expand from the inside like most application sidebars, are used. Drawers were very common in early versions of Mac OS X. Mail, which used drawers for listing mailboxes, lost them in 10.4 ("Tiger") and saw them replaced by a traditional sidebar. A number of other Apple-created applications and third-party applications have replaced drawers with a sidebar, or re-designed the interface to make a sidebar/drawer unnecessary. Formerly drawer-heavy apps, like iCal and Adium, now contain no drawers at all, and instead display an optional sidebar within the main window.
Examples of apps with drawers include: