The simplest UX Tool is still pen and paper. 

If you want to add sharing screen capture and share through any cloud service. Experience Designers have been looking for tools that organize data, the core organization of data is dependent of the project at hand. The process to make the most valuable and unique solution, it is also based on the project or challenge at hand. 

Unique experiences will demand the most versatile, playful, quick and explorative tools; thus nothing can be as fast and swift as hand and paper when creating personas, rapid prototyping and sketching ideas, which is one of the main reasons UX Designers use and discard many software tools. We keep going back to basic and minimal digital solutions, which were not mentioned, like Paper, Mural, Pop, UXPin Paper Notepads, even overpriced Moleskine Sketchbooks and Field Notes Pads. However, a napkin or sheet of paper combined with any pen or pencil can be the most versatile tools for all designers and creatives, and what a mighty mix if you throw-in a few Post-Its.