NetAlertX is described as 'Network intruder and presence detector. Scans for devices connected to your network and alerts you if new and unknown devices are found' and is a Network Monitor in the network & admin category. There are more than 10 alternatives to NetAlertX for a variety of platforms, including Windows, Linux, Android, Mac and iPhone apps. The best NetAlertX alternative is Nmap, which is both free and Open Source. Other great apps like NetAlertX are Angry IP Scanner, Fing, NetPeek and NETworkManager by BornToBeRoot.