FindMe by SMS

Replies with an SMS that contains GPS or CellID co-ordinates and a link to website that will show the Google map with that location marked.

Works in background and does not alert user that their location is queried. Restarts automatically if phone is power cycled.

The incoming SMS message must have this keyword:

fm?

If PIN protection is enabled the PIN needs to be specified in the incoming SMS message as well, eg:

fm?1234

If GPS is not available then app replies with link that includes the CellID information, and clicking on the link in the SMS message will bring up a map showing location of the phone tower with which the phone is communicating.

Free.
Simple to use.
Only starts when SMS received, and stops running after sending response. Does not use any power otherwise.
Developed on HTC Touch Diamond 2, so not sure how the admin screen will look on lower res devices...

FindMe by SMS