Protractor

Developed for the Yahoo! Widget Engine, Protractor is a small widget that provides you with a simple tool for measuring angles and circles, directly from your desktop.
It can be used by math students for upcoming geometry projects, for instance. The app is easy to install and set up, regardless of the user's experience level.
The interface of the tool is based on a large, transparent frame that you can move to any position on the screen, with the help of the mouse cursor.
By opening the context menu, you can set the relative rotation, angle marker and compass deviation. Additional settings are available in the Widget Preferences screen. For instance, you can also mark the zero degree.
Regarding the frame's appearance, you can adjust the protractor's size, background and compass opacity, as well as establish the background size, angle marker color, compass style (default, four-point, eight-point) and colors. Thanks to the default options provided by the Yahoo! Widget Engine, you may also make the frame stay on top or below all the other windows.
The lightweight tool barely uses CPU and system memory, so it doesn't put a strain on the computer's overall performance. It has a good response time and works well, without causing the operating system to hang, crash or display error notifications; we haven't encountered any issues throughout our testing. All in all, Protractor is a very reliable tool for measuring circles and angles in a virtual environment.