Some of very useful tools from Windows. Some of these have been vital to be vital in the past years.
Rating:
* Good
** Good and unique (not many tools offering these functions on Windows)
*** Vital, could not have done my job without it.

System:
Filemon by Sysinternals (Now Microsoft):  ***
This tool is critical to system administrators. This tool basically will list, in a GUI, the files being accessed in real time on your server. You can use that information for many troubleshooting tasks (high CPU, something weird happening on the server....etc...). This tool will show when a file access is being denied because of a lack of permissions. This is very useful when you run Services under a restricted user account (versus the default System) and you application does not work. With this tool you can slowly build a list of the files and folders that will require permissions. Note: I've used FileMon without any issue for years on Windows 2000, recently on Windows 2003, I've notice some occasional refreshing issue.

Regmonitor by Sysinternals:  *
Same as Filemon for the registry.

ProcessExplorer by Sysinternals (Now Microsoft):  *
Very comprehensive tool to inspect currently running processes.

CurrProcess by NirSoft:  *
Inspect currently running processes.

OpenedFilesView by NirSoft:  *
Inspect currently open files with many details.

Autoruns by Sysinternals:  *
Very comprehensive tool to inspect startups programs and more.

StartupRun by NirSoft:  *
Basic tool to inspect startups programs.

ServiWin by NirSoft:  *
Inspect and configure drivers and services.

Network:
Active Ports 1.4: **
This tools will list all the network connections, in a nice GUI. It's kind of like a graphical netstat command. This tool has not been upgraded since years but does the job. Note: Some Anti-Virus tools (like Symantec AV Corporate) detect this tool has a potential "hack" software. It is not unless you install a real "hack" program that uses the API Active Ports offers.

TCPView by Sysinternals (Now Microsoft):  *
Detailed listing detailed listings of all TCP and UDP endpoints on your system.

CurrPorts by NirSoft:  *
Displays the list of all currently opened TCP/IP and UDP ports on your local computer. Just started using this tool.

SmartSniff by NirSoft:  **
Capture TCP packs in real-time. Just started using this tool, it's very nice so far.

IPNetInfo by NirSoft:  **
Find all available information about an IP address. This tool will save you some time when you try to track down ips on a regular basis.

Security:
AccessEnum by Sysinternals:  *
Quickly list the folder or registry access permissions.

RootkitRevealer by Sysinternals:  *
Tool to detect root kits.