WinMerge - Open source differencing and merging tool for Windows
HeidiSQL - Open source frontend for MySQL, MariaDB, PostgreSQL and Microsoft SQL Server
Data center information management
Netbox - Open source and web-based IP address management (IPAM) and data center infrastructure management (DCIM) tool.
Claws Mail - Open source email and news client for Windows and UNIX like systems
Integrated Development Environment
NetBeans IDE - Software development platform written in Java which supports Java, PHP, C/C++ and HTML5
Wireshark - Open source packet analyzer for network troubleshooting
XCP-NG - Turnkey Open Source Hypervisor, based on XenServer
LibreNMS - LibreNMS, a fully featured network monitoring system that provides a wealth of features and device support
Gpg4win - Open source email and file encryption for Windows using GnuPG public-key cryptography
KeePass - Open source password management tool
Privacy Badger - Spy Ad and invisible tracker blocker by EFF
mRemoteNG - Open source, tabbed, multi-protocol, remote connections manager
WinSCP - Windows Secure Copy, open source SFTP, FTP, SCP client for Windows
PuTTY - SSH and Telnet client for Windows
WiFi network scanner
Homedale - Not open source but free to use WiFi network scanner/monitor
iperf - Measure bandwidth and performance of a network
nmap - Port and security scanner including host discovery and OS detection
skipfish - Web application security reconnaissance tool
Debian Gnu/Linux - Debian is a free and open source operating system.
FreeBSD - FreeBSD is a free and open source operating system based on Berkeley Software Distribution.
OpenBSD - OpenBSD is a free and open source operating system based on Berkeley Software Distribution.
OpenIndiana - OpenIndiana is a free and open source operating system derived from OpenSolaris.