Nmap - Nmap Free Security Scanner, Port Scanner, & Network Exploration Tool. Download open source software for Linux, Windows, UNIX, FreeBSD, etc.
Neofetch - Fancier version of Screenfetch that displays colors and supports more platforms.
Wireless Network Watcher - scans your wireless network and displays the list of all computers and devices that are currently connected to your network.
Archey 4 - Archey 4 is a system information tool written in Python
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.
Speccy - Speccy - find the details of your computer's specs. Great for spotting issues or finding compatible upgrades. Download the latest version free.