Sysinternals is run by Mark Russinovich and Bryce Cogswell, who are two IT industry "heavy weights".  Sysinternals provides a lot of awesome, free utilities for system administration and troubleshooting. 

Some of my favorites are FileMon, RegMon, and the PSTools suite.  I've used FileMon on many occasions to troubleshoot application errors only to discover that the errors were due to insufficient file permissions.  RegMon can be used for the same thing but for registry access instead of file access.  The PSTools suite has several handy utilities including PSShutdown which i use for scheduling reboots of servers.

I first learned about these utilities from Mark Russinovich's articles in the "Windows IT Pro" magazine.  To find out more, go to http://www.sysinternals.com/