I am a bit of a technophreak and one of my favourite devices is my handheld GPS. I use a Garmin 12XL unit like the one on the left here, which keeps me updated with positional information when I am traveling.
The GPS system is run by the Americans but is available for use by people all over the world at no cost except for the purchase of a suitable GPS unit.
As far as us ordinary mortals are concerned a GPS unit is capable of giving positional information that is accurate to better than 100meters 95% of the time. For the military and other specialist users the accuracy of GPS is far better.
As a walker I have always relied on being able to use a map and compass and the advent of GPS has not changed that. The GPS just makes navigation easier in some ways and certainly more fun for someone like me who quite unashamedly likes to own and use technical gadgets. However, knowing how to use GPS is no substitute for knowing how to use a map and compass and I personally would not dream of doing any serious walking without the essential support of these two old faithfuls :)
I owe a debt of gratitude to the wonderful website of Joe Mehaffey and Jack Yeazel. For anybody who wants to know anything about GPS this has to be the essential place to start. Another useful place for help on GPS related matters is the news group news:sci.geo.satellite-navigation.
The GPS manufacturers also provide excellent sources of information about GPS in general and also specific information and support for their own products. Trimble in particular have some excellent resources while Garmin make some of the most popular models on the market today. Another excellent place for Garmin related info' is Tom's Garmin GPS Pages
One of the most useful accessories to add to a GPS is some type of mapping software. This allows you to plan routes on a map, mark waypoints etc. and then upload the whole lot into the GPS. The alternative to this is to tediously enter every grid reference keystroke by keystroke! There are several packages about that will do this, and more , but my personal favourite is OziExplorer. This is available as a facilities restricted trial version with the full package available on payment of a very modest fee.