A free app for Android, by Inventive Crafts.

If you have a hunch that someone is leeching off your Wi-Fi, then this app is the best for you. 

Wifi Detector is a tool for wireless network security and for finding out who is using your network and who is not. If you want to know who is connected to your wireless network, you can use this tool to check the data. With this tool, you will be able to check the IP address, device name, manufacturer, and Mac address.

This app is available on Google Play Store.