Total Network Inventory: Professional PC audit and network inventory management software - fits best for a for companies of all sizes. Free for 60 days.
The Dude - MikroTik is a Latvian company which was founded in 1996 to develop routers and wireless ISP systems. MikroTik now provides hardware and software for Internet connectivity in most of the countries around the world.
Network Inventory Advisor - Network Inventory Advisor audits all network software and hardware simply and efficiently
Pandora FMS - Pandora FMS: Opensource monitoring software
Lansweeper - Lansweeper discovers Windows, Linux and Mac machines in your network.
OpenNMS - Project home page of OpenNMS - a java based network managememnt platform developed under the Open Source model
Nmap - Nmap Free Security Scanner, Port Scanner, & Network Exploration Tool. Download open source software for Linux, Windows, UNIX, FreeBSD, etc.