AirSnare is a free, useful and reliable intrusion detection system to help you monitor your wireless network.
It notfies you as soon as a machine connects to your wireless network, that is not included in your list of friendly MAC addresses.
You can configure the program to send an email notification to the administrator, log the connection, and also send a message to the connected machine, informing them that their actions are being monitored.
AirSnare integrates with Ethereal for additional package capturing features. Most desktop 10Mbs and 10/100Mbs should work if using AirSnare on a wired network (see the How to setup document).
AirSnare is a completely free program. Creating and maintaining AirSnare has required - and still does - a considerable amount of work and effort.
If you like AirSnare or you are using it for a commercial application, then you may wish to support AirSnare development by making a voluntary donation.