Once in a while friends and relatives would visit my family from other countries, and we usually go to the airport to pick them up. The problem is that sometimes flights are delayed, and we would end up driving around the airport for half an hour or pay for expensive airport parking. Fortunately, there are several sites where you can check for airport conditions and whether or not a certain flight is delayed.
First, you need to know the airline and flight number you are waiting on, and then you can go to one of the following sites to check its arrival and departure statuses.
Flightview.com - This site has the least amount of advertisement and is very easy to use. You can pick the airline name in a drop down and then type in a flight number, then it will show you the status of the flight. They also have a mobile flight tracker for phones. This site also has an airport departure status of major airports in the continental United States.
Flightstats.com - This site is actually integrated with Google. So if you search for the flight number in Google the status will show up immediately at the top. Here is an example:

If you go to Flightstats.com you will find more detailed information and an interactive map for airport statuses all over the world. They also have a delay rating for each flight.
Flightarrivals.com - I think this is the oldest site of the bunch and it has been around for around 10 years. It is not very fancy, but it loads quicky and it is simple.
I have found that the most efficient way to use these sites is to check if my guest's flight arrived on time, and then head to the airport. It usually takes at least another half an hour for a person to actually leave the plane and get out of the airport with their baggage so I always arrive just a little after they are out of the gates.
Saving time is equivalent to saving money, so before you head out to pick up someone at the airport next time, run a quick check if their plane is on time. You could avoid quite a bit of frustration and wasted gas.