Mr.Muh

With the following few lines of code you can read the framebuffer (OpenGL) of your choice and export it as an image (e.g. PNG). You will the DevIL Library that could be found here

// allocate array (take care about the number if channels >> for RGBA use width * height * 4!)
GLubyte* fboData = (GLubyte *)malloc(sizeof(GLubyte)*windowWidth * windowHeight * 4);

// bind screen buffer or the buffer you want
glBindFramebuffer(GL_FRAMEBUFFER, 0);
glReadPixels(0, 0, windowWidth, windowHeight, GL_RGBA, GL_UNSIGNED_BYTE, fboData);

ILuint imageName;
ilGenImages(1, &imageName);
ilBindImage(imageName);
ilTexImage(windowWidth, windowHeight, 0, 4, IL_RGBA, IL_UNSIGNED_BYTE, fboData);
ilSaveImage((ILstring)"YOURFILENAME.png");
// free DevIL image
ilDeleteImages(1, &imageName);
// free array
free(fboData)

Thanks to Infamous for the hint!