Since you can't reserve a seat ahead of time, getting an "A" boarding pass turns out to be critical. If you wait until you arrive at the airport to check in, you'll almost certainly be stuck with a "B" or even a "C", and even if you wait more than an hour at the front of the line for your particular boarding group, you still risk getting stuck with an aisle seat. In some instances, you apparently have to check in within the first few minutes (i.e., exactly 24 hours before your flight time) in order to get an "A" boarding pass.
BoardFirst and similar services have seemingly found a way around this ridiculous situation. (Yes, I realize that some people actually like Southwest's dysfunctional approach; I think it's just because they've figured out how to beat it, kind of like how ticket brokers who have figured out how to get good concert tickets from Ticketmaster don't think that system is broken.) If you're willing to trust BoardFirst with your Southwest flight information, BoardFirst will log on to Southwest's website when check-in opens and put you near the front of the line for an "A" boarding pass. BoardFirst charges $5 for this service.
Southwest sued BoardFirst last year based upon a variety of legal theories, including a claim that BoardFirst enters into a browsewrap contract with Southwest each time it accesses Southwest's website on a customer's behalf, and that contract prohibits third parties such as BoardFirst from checking customers in or obtaining boarding passes for them. This month the court ruled in Southwest's favor on that claim, although from a quick look at BoardFirst's website, it appears that BoardFirst may still be accepting "orders" from Southwest customers.
Southwest Airlines Co. v. BoardFirst, L.L.C., No. 3:06-cv-00891 (N.D. Tex. Sept. 12, 2007).