Purchasing a GPS device for your bike is beneficial for a number of reasons.
Not only is it nice to know where you're going and how long it's going to take to get there but there is also the increased security of knowing that if you are in an accident or get caught in bad weather you will be able to readily identify your exact location.
It's also great for building up routes that you can cycle again and again and share with others
Attaching a GPS receiver to your bike is very straightforward with many manufacturers now selling bike mounts that you can simply attach to the front of your bike.
The models the mounts are designed for are generally fairly rugged, waterproof machines that can take a few bumps and nasty weather conditions without any problems.
Below are some of the most suitable GPS devices for bicycles.

Garmin Garmin are one of the leading manufacturers of GPS units with several receivers suitable for bike use.
The etrex series is particularly suited to all types of biking and can be purchased with a handlebar bike mount.

Magellan is another leader in the GPS market with an extensive range of outdoor devices to choose from.
The Explorist range contains a number of suitable devices with an easy to install bike mount a popular accessory