My favorite tool is asking for help. You may not think of it as a tool, but it’s something I use frequently to solve a wide range of problems, so I think it qualifies. Whether it’s saying, “I don’t know what to do next – does anyone have any ideas?” when teaching, or cold-calling people to see if they’ll host a workshop, asking for help has probably done more for me than Emacs and version control combined.
Have you got a favourite tool you would like to tell us about? Please use this form to add a bit of detail and we will do the rest. You can read the background to these posts here, or see what other tools people have written about.