One of the most popular questions asked of cell phone callers is “Where are you?”

Thanks to a new service, you may not have to ask or answer that question. Glympse, which launched on Monday, joins Loopt and Google Latitude as the newest location-based service that uses cell phones’ GPS capability to tell people where you are.

Glympse, which is free, has taken a different approach. Unlike Loopt, which requires the sender and recipient to sign up for service and download an application, Glympse requires almost no effort on the part of the person who is following you. All they have to do is click on a Web link on a computer or a Web- enabled phone to see where you are on a map.

To transmit your location, you need to download an application to your phone and use the application to send a “Glympse,” which authorizes that person to follow you for a specific amount of time and send them the link they need to see you on a map.

Another way it differs from both Loopt and Google Latitude is that Glympse can automatically time out after tracking someone for four hours. The person being followed can also choose a shorter monitoring window, like 30 minutes.

To its credit, Loopt is also permission-based and sends reminders that you may be sharing your real-time information. Google’s Latitude service, which also requires you to give permission to be followed, only displays your approximate location — within about a half mile — but doesn’t show precisely where you are.

With Glympse, you might not know the person’s exact address, but you’ll probably be within a hundred feet.

The coolest thing about Glympse is when you are following someone on the move. You can pinpoint them on the map, see their speed, and see when and where they stop.

I used it to follow Glympse co-founder Bryan Trussle as he rode in a car from my house to San Jose. I saw him get on and off the freeway and pull into a parking lot. At one point I caught his car’s driver exceeding the speed limit by a few miles an hour.

As he compared his actual location to what I saw on my PC, we found there only a slight lag between the two — a difference of 10 seconds on average.

While I realize that some will consider this technology a little creepy, there are practical uses for it.

Some police departments, delivery services and businesses spend a lot of money on equipment to do what Glympse could do for free.

For one thing, it can reassure parents and family members that their loved ones are safe.

I remember how nervous I was when my 16-year-old daughter would use the car or ride in other kids’ cars. We’d insist that she call us when she arrived and made her tell us where she planned to go. If she had a Glympse-equipped cell phone, we could have checked in on her from time to time — and we would have known if she was speeding too.

And rather than calling people to let them know you’re arriving in 15 minutes, they can track you online or on a Web-enabled phone.

The Glympse’s public beta currently works only on the T-Mobile G1, which uses Google’s Android operating system. The company is working on versions for the iPhone, Windows Mobile and BlackBerry.

Disclosure: Glympse, as well as its main competitors, Loopt and Google, are supporters of ConnectSafely.org, a nonprofit Internet safety organization I help operate.