Corbit is a C++ port of the QB version of orbit, perpetrated by the dmags, while involved in OCESS (spacesim.org). This means it is a spaceflight simulator in 2D, with basic graphics, and a focus on comprehensive data readouts. It is also designed to be easily maintainable, quickly expandable, and simple to fly.
And the canonical pronounciation is korbit.
Read the docs for more info. At the moment this is a monopoly project, but take a glance at the source and tell me what you think.
To build this, you need allegro 5.1 (preferably), boost libraries, the Eigen3 vector libraries, jsoncpp, and C++11