Update: 2015/12/15 - Reorganize categories, add some PowerShell links

Tools

SYDI - Network Documentation made easy (VBS script) SYDI-Server v.2.3 Server is a tool for documenting Windows computers
vCheck - is a PowerShell HTML framework script, the script is designed to run as a scheduled task before you get into the office to present you with key information via an email directly to your inbox in a nice easily readable format.
SysInternals - Microsoft Sysinternals utilities help you manage, troubleshoot and diagnose your Windows systems and applications.
mRemoteNG - is a fork of mRemote, an open source, tabbed, multi-protocol, remote connections manager. mRemoteNG adds bug fixes and new features to mRemote.
JoeWare Tools - offers a bunch of tool for saving administrators around the world time and frustration
NirSoft Tools - NirSoft web site provides a unique collection of small and useful freeware utilities, all of them developed by Nir Sofer.
CJWDEV Tools - offer softwares aimed at helping make the jobs of fellow IT Professionalss easier
NetPerf - Netperf is a great tool for obtaining instantaneous network bandwidth between a client and server. Instructions on using Netperf can be found in this article
Netwrix Freetools
WMI Explorer 2014 (SAPIEN) - Allows you to get a handle on WMI classes and their properties and methods
WMI Explorer (PowerShell Script) - PowerShell-based WMI Explorer tool created by Marc van Orsouw (aka /\/\O\/\/)
WMI Explorer 2.0 (From Microsoft senior support escalation engineer, Vinay Pamnani)
Chocolatey - Chocolatey NuGet is a Machine Package Manager, somewhat like apt-get, but built with Windows in mind.
BoxStarter - Repeatable, reboot resilient windows environment installations made easy using Chocolatey packages. When its time to repave either bare metal or virtualized instances, locally or on a remote machine, Boxstarter can automate both trivial and highly complex installations. Compatible with all Windows versions from Windows 7/2008 R2 forward.
Hp ProLiant Memory Configurator - Online tool to help you configure the memory on your physical servers
NSSM (the Non-Sucking Service Manager)
SCSM Entity Explorer
Run-virtual.com Tools - Set of vSphere tools created by Richard Garsthagen
RvTools - RVTools is a windows .NET 2.0 application which uses the VI SDK to display information about your virtual machines and ESX hosts
Ethtool - Display or change ethernet card settings

One of the feature of this tool, allows you to make the NIC blinks! This come very handy when you need to identify which NIC is which VMNIC [Article] for example: ethtool -p vmnic0 5 will make the NIC blink for 5 seconds.

Esxtop - VMware's command-line tool for analyzing vSphere performance issues.
Veeam FastSCP - Fast Copy between ESX host
PuttyTray - PuTTY Tray is an improved version of PuTTY. It features some cosmetic changes, and a number of addons to make it more useful and much more fun to use.