Sysinternals is a site run, created and maintained by Mark Russinovich and Bryce Cogswell. It includes a good array of free tools, source code and technical information useful to those exploring Windows itself or those wanting to monitor what a program is doing whilst it is running. This is something hackers and crackers do so you should be aware of the tools that hackers could use. Of course most if not all of these tools are also valid options for general monitoring and maintenance..:-)
The site includes a very impressive array of utilities including file and disk tools, security tools, networking tools, process and thread tools, system information tools and miscellanious tools. There is also a lot of source code for utilities, information, forums, newsletters etc. This is again a superb site for the depth of information and the large collection of free tools.