An app I have been using for several years is Glympse. It is very close to perfection.
It lets people know where you are. A common example is if you are meeting someone somewhere they can monitor your progress and estimated time of arrival. This is especially useful while driving.
Where Glympse gets things right is how smooth they make it. Unlike most other apps, absolutely no authentication, account creation, social media profiles or privacy invasion is required. That applies to both the sender and the recipients. Glympses automatically timeout (your choice of up to 4 hours, or on reaching your destination).
The Glympses themselves are a URL in the form glympse.com/123A-BCD4 which is sent to the recipients. Providing they can access the web, they can see details. You are tracked on a moving map, along with updated ETA, speed (if you chose to share it), traffic and track. Recipients on mobile devices can use the Glympse mobile app to track multiple people at once etc, but this is not required.
Gympses can be sent in whatever methods you have made available. For example SMS can be used on mobile, Twitter if you logged in, email etc - all at once.
The pervasive feeling throughout is an app on your side. Every potential source of friction has been removed, for both senders and recipients. You don't feel like a pawn in some ecosystem or social network war. This stuff just works, and works well. Recommended.