Let us share some tools that are either free or have a generous free offer that you cannot believe its free (without self-promotion please).
For me it will be:
https://www.putty.org/ (protocol terminal tool)
https://www.7-zip.org/ (WinZip alternative)
https://trello.com/ (project management with generous free tier)
https://filezilla-project.org/ (FTP client)
https://github.com/gorhill/uBlock (best ad blocker - uBlock Origin)
https://www.voidtools.com/ (for its Everything tool, that is a must-have for Windows users)
https://www.google.pl/maps/ (I think most of us take for granted this one)
https://duolingo.com/ (awesome for learning new languages)
https://notepad-plus-plus.org/ (it just works)
Can you list more?