FASTpass

FastPass is one of the best tools for saving time on your Disney vacation.  With the swipe of your park ticket, it gives you a free "pass" to get in a shorter line later that day.  During crowded park days, this one little object can get you on twice the attractions.  FastPass does come with rules though.  Printed on your FastPass is a return window of one hour for you to come back to ride.  You also can't get another FastPass until the return window opens up OR 2 hours have passed from the time you got your current one, so it takes a little planning to use them well.
Recently, Disney started getting much more strict on the return window of your pass.   

One way to really utilize the system is to get your FastPass, like the one above, for the most popular ride early in the day.  The earlier in the day you get a FastPass, the more likely you will have a return window within a couple of hours.  Right before your window to ride opens up, plan to be near another FastPass attraction, so you can get your next FastPass.  Then head over and use your current pass. 
Never get a FastPass for anything with less than half an hour wait, as you won't be saving that much time, as there are still sometimes about 15 minute waits in the FastPass line.  Of course, if you plan to ride something twice, go ahead and grab one because there might be a line later in the day. 

FastPass is free to all Disney park guests.  If you plan accordingly you can get the most out of your time at Disney, and you'll spend less time waiting in line.