Please note, this is a live page that will be added and subtracted from as time goes on, please visit regularly for any changes I make.
Sysinternals (sysinternals.com)
Sysinternals tools is a compilation of over 70+ Windows-based tools. Most of these tools will save your proverbial IT Admin life at one point in time. I would HIGHLY recommend you add the folder for these tools into your %PATH% so that you don't have to manually find their location every time you want to use one. Here are a handful I find myself using quite frequently:
PSLOGGEDON - See who's logged into a particular PC, although I've recently migrated to qwinsta and rwinsta
PSEXEC - I mostly use this to get a remote CMD connection on another PC but you can use this to remotely run scripts or other items on a remote computer without needing to RDP into them (or talk to users)