I'm currently using Prey which seems to be the best free software that I've seen. It provides free protection for 3 devices and meets the requirements found within the question. It's available for many devices including desktops, laptops, tablets, and phones.

# [Prey](https://preyproject.com/) (open source)

> Prey is a lightweight application that will help you track and find your laptop or mobile if it ever gets stolen. It works in Windows, Mac OsX, Linux and Android. 

> [![enter image description here][1]][1]


  [1]: https://i.sstatic.net/bklpu.jpg