Scan your Wifi with Fing

I'll suggest an app that I discovered while investigating with the Arduino Ethernet Shield, it's useful to scan the wifi your phone is connected to, it will show you all devices connected to the same router, as well as its local IP and MAC, so you'll be able to redirect ports, and other stuff

Completely free

Link to: Apple Store

Link to: Google Play

Images:

iPhone Version                                                          Android Version