Zabbix - Track, record, alert and visualize performance and availability of IT resources
Nmap - Nmap Free Security Scanner, Port Scanner, & Network Exploration Tool. Download open source software for Linux, Windows, UNIX, FreeBSD, etc.
Abusix - Abusix is a network security company that offers email threat protection and network abuse handling solutions to ISPs, Telcos, and Cloud/Hosting Providers to stop the rapidly growing problem of email-borne threats and other kinds of network abuse.
Angry IP Scanner - Open-source and cross-platform network scanner designed to be fast and simple to use
FastNetMon - FastNetMon is a very fast DDoS analyzer with sflow/netflow/mirror support.
Advanced IP Scanner - Advanced IP Scanner shows all network devices, gives you access to shared folders, and can even remotely switch computers off. Download it Free.