Google Latitude allows you to see you and your friends on Google maps on a phone or on a computer. Friends can be found by browsing Gmail contacts and introduced via SMS on a phone, or just used directly with some common Google services, like iGoogle on a regular computer. It’s currently ‘available soon for iPhone and Android’ — i.e. I can’t test it yet. For those of you that have, let me know your experiences.
You get good control over with regards to your location privacy — just like on an IM service, you can choose to be invisible.