The Garmin Edge 510 and 810 models come with an automated GPS based tracking tool and include an iPhone/Android app that connects to the Edge unit via Bluetooth. It broadcasts updates every 30 seconds or so. The user specifies who can see it from the phone’s contact list. There are also options for Twitter and Facebook. It sends the watchers a link to a web page. The web page includes a map, overlaid with a trace of your progress, and a select set of values of ride performance, including average speed
I have a SPOT tracker. The advantage of the spot is that you don't need cellphone coverage. It's amazing how often you don't have cell coverage. Also, the current version doesn't use batteries very fast and you can use just about any AAA battery.