Logseq for notes and "Lazy GM" checklist, GIMP for carving out maps from the ones I have, Tiled to populate them. Tarot (I mean the cards, not some software) for populating initial background connections when I'm completely stuck

EDIT: I know that this will be unpopular here but I also use Mistral to convert my dry three sentences into radio speaker text with the right angle or mail exchanges my players "might" find on the host while hacking