Being somewhat cheap thrifty, I didn't want to spend the money to buy an official cable for my Magellan Sportrak GPS. After a little work with google, I found this site that deals with the Magellan 315, which happens to have the identical connector to the Sportrak. (Most all of the Magellan GPS units have the same connector, both physically and electrically)
So I made my own, using an old serial cable, a peanut butter lid (yellow, to match the GPS ), a couple of pen springs and LOTS of hot melt glue.
The big improvement I made was in the double layer plate, which allows the springs to compress into the plate slightly, and allowed a nut on the screw to make it captive.
DIY Magellan GPS Cable
Combining that with the free EasyGPS software and *boom*, good to go. Sure is simpler than typing waypoint info using the up/down/enter mechanism of the GPS itself.