Total Network Inventory - Total Network Inventory is a piece of software designed to help with managing software inventory and auditing PC systems.
Wireless Network Watcher - scans your wireless network and displays the list of all computers and devices that are currently connected to your network.
Lansweeper - Lansweeper discovers Windows, Linux and Mac machines in your network.
Nmap - Nmap Free Security Scanner, Port Scanner, & Network Exploration Tool. Download open source software for Linux, Windows, UNIX, FreeBSD, etc.
Spiceworks Help Desk - Help desk software features that work for IT
Angry IP Scanner - Open-source and cross-platform network scanner designed to be fast and simple to use