some basic tools (free)

easy text editor: textpad

graphics: paint.net (needs some plugins though, for example to support webp)

file search: agent ransack

analyze drive size/contents: treesize free

free SSH client for Windows: PuTTY
more advanced (tabs, credentials cache): Solar PuTTY (free, but beware sales team like a stable fly)

ftp client: filezilla

local WAMP development environment: wamp64

tool to compare files and generate patches: winmerge

zip and unzip: peazip or 7zip (although I found 7zip to corrupt files in some instances)

free server administration interface/panel: ISPConfig

synchronize outlook with google calendar: Paul Woolcock's outlook google calendar sync: https://github.com/phw198/OutlookGoogleCalendarSync
this allows to synchronize a locally installed outlook calendar with your phone's google calendar. very useful.