Is there any opensource software that controls the RAMPS RepRap board directly from the computer?
No, there exists no software like that. You can't communicate with a RAMPs board because a RAMPs board has no logic built-in; it's just a dumb breakout board that connects the Arduino Mega to your printer's components (such as stepper drivers, MOSFETs for controlling heaters, endstops,...).

If you wanted to connect a RAMPs board "directly" to your computer, you'd need a way for your computer to generate the pulses that the Arduino normally generates. You could potentially use a parallel port for this but you'd need many more outputs than a single parallel port can provide and you'd need to find a way to do the analog to digital conversion needed for the thermistors.

It's just not very feasible (considering how few computers have parallel ports nowadays).