There are two types of extendable SmartDashboard elements: Widgets and StaticWidgets. Widgets show up on the dashboard in response to code on the robot calling SmartDashboard.put() (or putDouble(), etc.); I don't think many teams created this type of widget during the 2012 season because it wasn't well documented and was pretty entrenched in the overall SmartDashboard system. However, there seems to be new documentation regarding them (
here), so I think they will be more used in 2013. StaticWidgets are not made in response to data, and are instead added to the dashboard manually. The CameraExtension that many teams based their image processing on was a StaticWidget, and other StaticWidgets can be found under View->Add.
See also the
official SmartDashboard documentation wiki and the
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