A useful rule to know if you're traveling by plane in the US. If you are flying on a commercial airline, and your flight is delayed due to a fault of the airline, Rule 240 is a get out of jail free card.
Originally, Rule 240 was an actual federal requirement -- before airline deregulation in 1978, carriers were required to tell the federal government exactly what they would do for the passengers if a flight was delayed or canceled. Nowadays, 240 just refers to the conditions of carriage filed with the U.S. Department of Transportation. They aren't required to have as strict fallback plans as they once did, but most carriers do keep to the old rules. They differ from airline to airline, and they don't apply if the flight was delayed of canceled due to force majeure events -- acts of god such as bad weather, strikes, riots, wars, governmental actions, or other factors outside of the airline's control.
But if your flight is late or canceled due to a mechanical error, poor scheduling on the airlines' part, etc., you usually have the 'right' to be moved onto another flight, even one on a competing airline. The usual contract of carriage promises to deliver you to your destination within two hours of the promised time, at no extra cost to you. This could mean a bump up to first class, a paid fare with another carrier, or just that the airline will try to bribe you into submission with vouchers for free hotel rooms and meals. Some airlines promise nothing more than the cost of your ticket back, but may still go further for the sake of public relations.
Airlines don't tell you about the 240 rule, but they all know of it. It is often referred to as a 240, and can be used as a verb ("we need to 240 him") or a noun ("we've got a 240 here"). You should find a copy of your airline's 240 policy -- the Department of Transportation requires copies of the policy to be kept available for the passengers at the ticket counter, but don't count on this. You should be able to find the conditions of carriage on the carrier's web site. The section that you want will probably be labeled "Failure to Operate as Scheduled" or some such.
The Travel Detective, by Peter Greenberg. Greenberg recommends checking up on your flight even before you leave for the airport -- and check not just on the scheduled flight, but also the aircraft. If the plane isn't on schedule (if it is still stuck in London three hours before you're supposed to take off in it from San Francisco), then you know the flight will be delayed. The flight will still be scheduled to leave on time, but you know it won't happen -- so you can ask for a preemptive 240. Good book. You should read it.