My favorite free operating software utilities come from Sysinternals. I just noticed on a blog post by the companies founders that they have been bought by Microsoft. I have mixed feelings about this news as I am a big fan of the software and the free-ness of it and am not sure what will happen to it in the hands of Microsoft.
On the plus side of this transaction is the fact that the creators of these great utilities will not be working directly for Microsoft so they should be able to have influence in keeping the code tight and features to admins rich. But on the bad side we will probably not see any new utilities from Sysinternals and with Vista coming who knows how well the apps will continue to work.
If you have not browsed through already you should not take a look at these free, mostly command line, utilities from Sysinternals and grab what you need. These are fantastic products that have saved me countless hours in diagnosing and troubleshooting Microsoft Windows machines both at the workstation and server level.