What are some of the preferred methods or libraries for creating video from an OpenGL graphics simulation?

For example, I want to create a visualization(video) of an N-Body gravity simulation by rendering non-real-time OpenGL frames.

The simulation is already coded, I just don't know how to convert it to video.

EDIT: I am also interested in providing the described functionality: The user can adjust parameters including the time step between captured frames and then initiate the simulation. The user waits for the simulation to complete, and then can watch the results. The user is able to increase or decrease the playback speed of the simulation whereas in slow motion, more frames are used i.e., you see higher resolution time steps, and when the speed is increased, you see lower resolution time steps at a higher rate, but the frames per second flashing on the screen is constant.