How many times have you tried to open a file in Windows, only to see it is already in use by some another program? Thus begins the witch hunt of process destruction, until you can open the file.
Process Explorer gets rid of this problem. Just search for a file name, and it tells you which process has a hold of it. This is only a small piece of its functionality, but a huge piece of my sanity. Why doesn’t Windows Ship with all the SysInternals stuff?