You can standby for free for the same routing (or a non-stop) on the same day of travel. If you arrive a couple hours early, you can standby to travel on the 10am ORD-IAD flight instead of the 12pm ORD-IAD flight for example. If they think you'll have problems or weather looks difficult, they'll probably rebook you/confirm you immediately on the non-stop. Ask at check-in and see what they'll do for you. You probably won't be able to standby for the non-stop as if you didn't make it, you won't have time to get to IAD to catch your regular flight. That doesn't mean that won't automatically confirm you on it if there is space available.