I use a cool combination of an iPhone and an application called Firepin to map my rides. Firepin uses the iPhone's GPS to locate and sends the data to the Internet, allowing you to be mapped in near real time.
Drawback: uses lots of power on the iPhone, I have to charge every couple of hours. Also, I can't see the data real time; only after I stop. A properly calibrated cycle computer is both visible real time and more accurate (the iPhone GPS records every few minutes or so, and assumes you've been moving straight in that time, which causes some errors in urban street situations). However, mine has been jinxed - first there were battery problems, then the removable unit got stolen.
Anyway, here are the last two rides mapped: