Total Network Inventory: Professional PC audit and network inventory management software - fits best for a for companies of all sizes. Free for 60 days.
Network Inventory Advisor - Network Inventory Advisor audits all network software and hardware simply and efficiently
TinyWall - Lightweight and non-intrusive firewall
Lansweeper - Lansweeper discovers Windows, Linux and Mac machines in your network.
SonicWall - SonicWall security solutions protects your network, systems, users and data from cyber threats.
Nmap - Nmap Free Security Scanner, Port Scanner, & Network Exploration Tool. Download open source software for Linux, Windows, UNIX, FreeBSD, etc.
pfSense - pfSense is a free and open source firewall and router that also features unified threat management, load balancing, multi WAN, and more