Administration

AD Manager Plus – A web-based very powerful AD-management tool. Loads of useful reports builtin.

VisionApp Remote Desktop – Tool to manage and access multiple remote servers and desktops

Scripting Tools

PowerGui – A must have tool if you are into powershell-scripting.

Quest AD CMDlets – CMDlets for easy powershell scripting management for Active Directory

SCCM

SCCM Right Click Tools – Excellent addon for SCCM Management Console.

Troubleshooting

Fiddler – Web proxy used for troubleshooting web-applications and more.

Memtest – To check memory modules on physical hosts.

Process Monitor - Tool by sysinternals. Used for advanced troubleshooting.

Wireshark – Network analysis tool.