Windows only: Freeware application DecaffeinatID is a simple intrusion detection and alert system for your PC that monitors your Windows logs for suspicious behavior.
DecaffeinatID will pop up an alert in your system tray whenever there’s an attempted remote login to your computer, and it detects changes in your firewall log, or your ARP changes.
The application is currently in beta and not a foolproof intrusion detection system by any means, but it’s also not a bad tool to run in your system tray next time you connect to the Wi-Fi at your local coffee shop. DecaffeinatID is freeware, Windows only.

DecaffeinatID [IronGeek via Hack a Day]