Is there any opensource software that controls the RAMPS RepRap board directly from the computer? I'd like to find an opensource software that communicates directly with the RAMPS board (or any other 3D printer driver) without using the arduino...

It doesn't matter if it runs on Windows or linux as long as it's opensource. Also, I'm not worried about how this communication is done (USB, serial port, parallel port)...

Has anyone ever heard about projects like this?
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).