Total Network Inventory - Total Network Inventory is a piece of software designed to help with managing software inventory and auditing PC systems.
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
Kuwaiba - Kuwaiba is an open source, enterprise grade network inventory and CMDB for your telecommunications and IT infrastructure
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.