Rain-X is a great product that we've been using on our car windshields.  It totally improves visibility when you are driving during wet weather.  I'm guessing it is oil-based, because you just apply to exterior glass and watch the raindrops fly off your windshield while driving. (Sap & bird poop doesn't stick as much either - big bonus!).  

Just spray it on and wipe it with a paper towel.  Takes a little time to rub it in, but well worth it (as it lasts quite a long while).  I also use it on my glass shower doors.  It totally reduces water marks and makes the water bead right up and roll away.