We have many great tools which helps a lot when programming, such as good programmers text editors, IDEs, debuggers, version control systems etc. Some of the tools are more or less "must have" tools for getting the job done (e.g. compilers).
There are still always tools which do help a lot, but still don't get so much attention, for various reasons, for instance, when they were released, they were ahead of their time and now are more or less forgotten.
What type of programming tool do you think is the most underestimated one? Motivate your answer.