Another vote for Google Latitude. Here are the limitations, though.
- The person being tracked has to explicitly add you to allow you to see them. They are also informed that you can see them, and they can shut down that consent at any time, set you to only see their location at the city level, or even enter a location they want you to see them at.
- The application only does near-real-time tracking when you are running Google Maps (in other words, the map is up on the screen). If the app goes into the background, it only gives very approximate location (nearest cell tower) and only once every 15 minutes minimum.
Google Latitude is more of a "I want to show my friends where I am when I want to tell them" sort of application than a "I want to be tracked 24/7" application. That's why it's legal to use.
But if you and your wife are (as in my case) just interested in a "are you on your way home from work?" or "when you're out kayaking I want to know your location from time to time" sort of thing, Latitude is perfect. It's not at all invasive, we both know we're running it, we both have it set so only the other can see location with any precision.
I only wish it had a setting that could "force-ping" my exact location at an interval I specify even when the screen on my phone is off. That way, I could set it to ping my location every half-hour, throw the phone in a drysack, and go out kayaking knowing my wife is getting occasional location updates.
I think there are other applications that do that sort of thing, I'm just not worried about it enough to do the research. Latitude meets most of my needs, it's free, and it's not invasive or creepy.