As a software developer, I use PerfMon and ProcessExplorer a lot. Especially useful when trying to figure out when something doesn't work.
Recently my team and I were trying to solve an issue with IIS AppPool because of high CPU usage. First think I thought of "Is there a tool which can take memory dump when these conditions occurs?". Then I checked Sysinternals and the tool was there, waiting for me. I somehow knew it will be there. Plus little bit of WinDbg, but that is different story
Thanks Mark for these great tools! They're making life a lot easier.