Nmap Free Security Scanner, Port Scanner, & Network Exploration Tool.

Advanced IP Scanner shows all network devices, gives you access to shared folders, and can even remotely switch computers off.

Terminals is a secure, multi tab terminal services/remote desktop client.

Nsauditor Network Security Auditor is a network security auditing software.

Zenmap is the official cross-platform GUI for the Nmap Security Scanner. It is free and runs on Linux, Windows, Mac OS X, etc.

SoftPerfect Network Scanner is a fast universal IPv4/IPv6 scanner with an extensive range of options and advanced features.

Nessus is a vulnerability scanner.