Cumulative Rendering 

glEnable(GL_BLEND);
glBlendFunc(GL_ONE, GL_ONE);
glDepthMask(GL_FALSE);
glBegin(GL_LINES);
for (int i=0; i < numParticles_; i++)
    {
    glColor3fv(particles_[i].color.vec);
    glVertex3fv(particles_[i].tailPosition.vec);
    glVertex3fv(particles_[i].position.vec);
    }
glEnd();
glDepthMask(GL_TRUE);
glBlendFunc(GL_ONE, GL_ZERO);
glDisable(GL_BLEND);

Example: