Process Explorer
It's only surprising to include a Microsoft tool until you realise that the whole sysinternals suite & Mark himself were borg'd by the Big M some while ago. The tools were developed externally to MS. And, indeed in my windows programming days I made extensive use of many of them and was very glad they existed, as there weren't many alternatives back then.
They were (and presumably still are) massive time-savers when developing software (that's what we did in the days before 'apps', kids! ;-)
<shuffles off to reminisce about DLL Hell to anyone who'll listen>