Freeware - PC Diagnostics
Windows Memory Diagnostic – http://oca.microsoft.com/en/windiag.asp
Windows Memory Diagnostic provided by Microsoft tests the Random Access Memory (RAM) on your computer for errors. The diagnostic includes a comprehensive set of memory tests. If you are experiencing problems while running Windows, you can use the diagnostic to determine whether the problems are caused by failing hardware, such as RAM or the memory system of your motherboard. Windows Memory Diagnostic is designed to be easy and fast. On most configurations, you can download the diagnostic, read the documentation, run the test and complete the first test pass in less than 30 minutes.
Fresh Diagnose – http://www.freshdevices.com
Fresh Diagnose is a utility designed to analyze and benchmark your computer system. It detects many kinds of hardware, such as CPU or hard disk performance, video or motherboard information, and much more. This software is 100% free, no ads, no spyware. WARNING: This site has annoying popups.
HeavyLoad – http://www.jam-software.com/software.html
Heavyload is intended to stress all resources of a PC (like CPU, RAM, harddisk, network, operating system, etc.) in order to test, if it will run reliable under heavy load. This is useful for testing important NT file or database servers before using them productively, or just to check if your new PC might get too hot when used intensively. To stress your PC or server, HeavyLoad writes a large test-file to the temp folder, it allocates physical and virtual memory and it draws patterns in its window.
Process Explorer for Windows - http://technet.microsoft.com/en-us/sysinternals/bb896653.aspx
Ever wondered which program has a particular file or directory open? Now you can find out. Process Explorer shows you information about which handles and DLLs processes have opened or loaded.
The unique capabilities of Process Explorer make it useful for tracking down DLL-version problems or handle leaks, and provide insight into the way Windows and applications work.
This application is really cool and useful!
WinAudit - http://www.pxserver.com/WinAudit.htm
PC audit and inventory of software, licenses, security configuration, hardware, network settings, etc. Fully documented, support command line, export to database, no setup (runs from a flash drive).
Magical Jelly Bean Keyfinder - http://www.magicaljellybean.com/
The Magical Jelly Bean Keyfinder is a freeware utility that retrieves your Product Key (cd key) used to install windows from your registry. It has the options to copy the key to clipboard, save it to a text file, or print it for safekeeping. It works on Windows 95, 98, ME, 2000, XP, Server 2003, Windows Vista (32bit and 64bit), Office XP, Office 2003, and Office 2007. Note that the last time I checked this site they haven't updated the software for awhile.