The Late Night Linux FamilyWhat's the best open source app or utility that no one else has heard of?
(Please note that your replies may be read out and discussed on an episode of Late Night)
Miguel BeltranOctoPrint is a must-have for 3D printers. It's a tool that runs as website, and can control your 3D printer using direct firmware messages.
Converts any cheap 3D printer into a smart one, you only need a Raspberry Pie or similar connected to it.