This applies only if you're flying southwest:
I just got a thing in the mail from southwest pointing out that 'You can now
check in online at southwest.com the day of your flight and receive your
boarding pass before you ever leave for the airport. To learn more, go to
southwest.com today!' I haven't tried it out, as I haven't flown southwest in
ages, but it would be pretty damn convenient if it's usable. (I assume you
might need to be a member of their frequent flier thing, but maybe not).