Southwest (WN) the mother (and father?) of LCCs used to allow this. However, too many were (ab)using this policy and they now state that you have to purchase a full-fare non-refundable ticket to standby on an earlier flight. Now, this policy makes no sense to me, because ideally you want to get people to their destination ASAP, maybe even earlier, so that you have a seat to sell for the later flight. On the other hand, people may want to purchase cheaper tix for later flights and standby for earlier ones, but if there's no space, nobody's at a loss.

BTW WN, unlike EZ, has a "flat-tire" rule: if you miss your flight, you can go out on the next one if it's within 3(?) hours of your original flight.