From Invoke Solutions:
Wifi Cop is a small window application that helps you monitor your home or small office wireless network. It will help you protect your network by keeping and eye and notifying you of any unauthorized access. This handy tool will also help you protect your data caps and limits by preventing rouge wifi users. It gives instant notification when a device enters your network, automatic scanning in the background, easy to setup/use wizard, 24/7 monitoring, automatically detects and identifies device manufactures, and automatic updates.