Total Network Inventory: Professional PC audit and network inventory management software - fits best for a for companies of all sizes. Free for 60 days.
Wireless Network Watcher - scans your wireless network and displays the list of all computers and devices that are currently connected to your network.
Network Inventory Advisor - Network Inventory Advisor audits all network software and hardware simply and efficiently
Nmap - Nmap Free Security Scanner, Port Scanner, & Network Exploration Tool. Download open source software for Linux, Windows, UNIX, FreeBSD, etc.
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
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.