A: Berkelium can be used anywhere you want to embed a web browser and need to control exactly how display or input are handled. As opposed to standard browser solutions, the browser is rendered off screen, into any buffer you like, and input is provided by you. In all other respects it is like a normal browser.
Berkelium can be used in a number of applications. Examples include embedding documentation into a game, using local web pages as UI elements, or even mapping browsers onto 3D objects to embed 2D pages directly into your 3D application. See Projects Using Berkelium for a full list of current uses.
A: Windows, Mac & Linux.
A: Berkelium should support plugins, but currently that support is limited to Flash (supported on all platforms). Development effort is not focused on supporting additional plugins because standards based approaches are preferred and Flash support covers a large fraction applications that require plugins.
For the curious with some spare time, it may be possible to use a hooked HDC on Windows, and to use the X11 Composite extension to render plugins to OpenGL on Linux. However, we suspect enabling arbitrary plugins and programs in this way would take a good month of work to get right.