Irrlicht Engine

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// Just a simple wrapper :D
struct IrrQuad {
	video::S3DVertex verts[4];
};

class CBeamNode : public scene::ISceneNode {
	private:
		// The beam material.
		video::SMaterial material;

		// Start/End Points
		core::vector3df vStart;
		core::vector3df vEnd;

		// Bounding Box
		core::aabbox3d<f32> Box;

		// Size of the beam
		float flScale;

		// Beam color
		video::SColor beamColor;

		void DrawQuad( IrrQuad& quad ) {
			u16 indices[] = { 0,2,3, 2,1,3, 1,0,3, 2,0,1 };
			video::IVideoDriver* driver = SceneManager->getVideoDriver();
			driver->setMaterial(material);
			driver->drawIndexedTriangleList( &quad.verts[0], 4, &indices[0], 4 );
		}

		// Thanks to whoever wrote this little function :)
		core::vector3df getTargetAngle( core::vector3df v, core::vector3df r) {
			//v -current node position
			//r -target node position

			core::vector3df angle;
			float x,y,z;
			x = r.X - v.X;
			y = r.Y - v.Y;
			z = r.Z - v.Z;

			//angle in X-Z plane
			angle.Y = atan2 (x, z);
			angle.Y *= (180 / 3.14); //converting from rad to degrees

			//just making sure angle is somewhere between 0-360 degrees
			if(angle.Y < 0) angle.Y += 360;
			if(angle.Y >= 360) angle.Y -= 360;

			//angle in Y-Z plane while Z and X axes are already rotated around Y
			float z1 = sqrt(x*x + z*z);

			angle.X = atan2 (z1, y);
			angle.X *= (180 / 3.14); //converting from rad to degrees
			angle.X -= 90;

			//just making sure angle is somewhere between 0-360 degrees
			if(angle.X < 0) angle.X += 360;
			if(angle.X >= 360) angle.X -= 360;

			return angle;
		} 

	public:

		CBeamNode( scene::ISceneNode* parent, scene::ISceneManager *mgr, s32 id, char* szBeam ) : scene::ISceneNode( parent, mgr, id ) {
			// Setup the beam material
			material.Wireframe = false;
			material.Lighting = false;
			material.MaterialType = video::EMT_TRANSPARENT_ADD_COLOR;
			material.Texture1 = mgr->getVideoDriver( )->getTexture( szBeam );

			// Default to 32 units for the scale
			flScale = 32.0;

			// Default to white
			beamColor.set( 255, 255, 255, 255 );
		}

		virtual void OnPreRender( ) {
			if( IsVisible ) {
				SceneManager->registerNodeForRendering( this );
			}
		}

		virtual void render( ) {
			// Figure out quads based on start/end points.
			core::matrix4 m;
			m.setRotationDegrees( getTargetAngle( vStart, vEnd ) );
			core::vector3df vUp( 0, 1, 0 );
			core::vector3df vRight( -1, 0, 0 );
			m.transformVect( vRight );
			m.transformVect( vUp );

			// Draw the first cross
			IrrQuad beam;
			beam.verts[0] = video::S3DVertex( vStart + vUp * flScale, core::vector3df( 1, 1, 0 ), beamColor, core::vector2df( 0, 1 ) );
			beam.verts[1] = video::S3DVertex( vStart + vUp * -flScale, core::vector3df( 1, 0, 0 ), beamColor, core::vector2df( 1, 1 ) );
			beam.verts[2] = video::S3DVertex( vEnd + vUp * -flScale, core::vector3df( 0, 1, 1 ), beamColor, core::vector2df( 1, 0 ) );
			beam.verts[3] = video::S3DVertex( vEnd + vUp * flScale, core::vector3df( 0, 0, 1 ), beamColor, core::vector2df( 0, 0 ) );
			DrawQuad( beam );

			// Draw the second cross.
			beam.verts[0] = video::S3DVertex( vStart + vRight * flScale, core::vector3df( 1, 1, 0 ), beamColor, core::vector2df( 0, 1 ) );
			beam.verts[1] = video::S3DVertex( vStart + vRight * -flScale, core::vector3df( 1, 0, 0 ), beamColor, core::vector2df( 1, 1 ) );
			beam.verts[2] = video::S3DVertex( vEnd + vRight * -flScale, core::vector3df( 0, 1, 1 ), beamColor, core::vector2df( 1, 0 ) );
			beam.verts[3] = video::S3DVertex( vEnd + vRight * flScale, core::vector3df( 0, 0, 1 ), beamColor, core::vector2df( 0, 0 ) );
			DrawQuad( beam );
		}

		virtual const core::aabbox3d<f32>& getBoundingBox() const {
			return Box;
		}

		virtual s32 getMaterialCount() {
			return 1;
		}

		virtual video::SMaterial& getMaterial(s32 i) {
			return material;
		} 

		void SetStartPoint( core::vector3df pos ) {
			vStart = pos;
		}

		void SetEndPoint( core::vector3df pos ) {
			vEnd = pos;
		}

		void SetBeamScale( float size ) {
			flScale = size;
		}

		void SetBeamColor( video::SColor color ) {
			beamColor = color;
		}
};

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CBeamNode* beam = new CBeamNode( smgr->getRootSceneNode( ), smgr, 200, "laserbeam.bmp" );
beam->SetStartPoint( core::vector3df( -500, 0, 0 ) );
beam->SetEndPoint( core::vector3df( 500, 0, 0 ) );
beam->SetBeamColor( video::SColor( 255, 0, 255, 0 ) );
beam->drop( );

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 CBeamNode* beam = new CBeamNode( smgr->getRootSceneNode( ), smgr, 200, "laserbeam.bmp" );
beam->SetStartPoint( core::vector3df( -500, 0, 0 ) );
beam->SetEndPoint( core::vector3df( 500, 0, 0 ) );
beam->SetBeamColor( video::SColor( 255, 0, 255, 0 ) );
beam->drop( );

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   vector3df zdir = end-begin;
   zdir.normalize();
   vector3df beamToCam = activeCamera->getAbsolutePosition().normalize();
//That's the important part. We want the "right" vector of the beam to be //perpendicular to the vector to the cam, so it will face as with maximum width.
   vector3df right = beamToCam.crossProduct(zdir);
   vector3df up = right.crossProduct(zdir);

   //No, you can't do that, but I would strongly suggest adding a constructor to allow exactly that instead of setting everything manually
   matrix4 mat(right.X, right.Y, right.Z, 0, 
                     up.X, up.Y, up.Z, 0,
                     zdir.X, zdir.Y, zdir.Z, 0,
                     position.X, position.Y, position.Z,1);
   setRotation(mat.getRotationDegrees());

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SceneManager->getVideoDriver()->setTransform(irr::video::ETS_WORLD, AbsoluteTransformation); 

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         //...
         material.MaterialType = video::EMT_TRANSPARENT_ADD_COLOR;
		 material.setTexture(0, mgr->getVideoDriver( )->getTexture( szBeam ));

         // Default to 32 units for the scale
         //....