What is the one piece of software you genuinely could not function without day to day

[Inlet]

Everyone has a piece of software that has quietly become load bearing in their daily routine, something they would genuinely struggle to replace if it disappeared tomorrow, whether that is a note taking app that has become an external brain, a password manager that has replaced years of insecure habits, or something far more niche and specific to a particular job or hobby that most people have probably never even heard of.

What makes a piece of software genuinely essential rather than just convenient is usually the sheer amount of habit and accumulated data built up around it over time, switching away often feels harder than the actual features would suggest purely because of how deeply it is woven into an existing workflow. So what is the one piece of software you personally could not function without, and how long have you actually been using it?