Total Network Inventory: Professional PC audit and network inventory management software - fits best for a for companies of all sizes. Free for 60 days.
Nmap - Nmap Free Security Scanner, Port Scanner, & Network Exploration Tool. Download open source software for Linux, Windows, UNIX, FreeBSD, etc.
Network Inventory Advisor - Network Inventory Advisor audits all network software and hardware simply and efficiently
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.
Lansweeper - Lansweeper discovers Windows, Linux and Mac machines in your network.
Angry IP Scanner - Open-source and cross-platform network scanner designed to be fast and simple to use
Port Authority - A handy systems and security-focused tool, Port Authority is a very fast Android port scanner.