Most of the answers on this site are made using programming languages, that are well known, and easily installable by most of the people. They have well known installers (or are already included) in all major PC operating systems. These includes languages like Java, Ruby or Python.
Some answers are in languages which are also widely available, but this might not be a well known fact. I'd include C# here, as some people think it's only available on Windows as a paid app (Visual Studio), but in reality there's Mono, which is free and available on all operating systems (including Windows), and there is also a free version of Visual Studio for C#.
There are also languages which are somewhat hard to obtain. For example Mathematica and Maple are paid apps without proper free alternatives. Matlab, while having Octave as a free alternative it's missing some features from Matlab you might be using. Or for example GTB only works on TI calculators, and the installation instructions on how to set up a TI virtual machine are hard to find.
So if you are more or less actively using a language which you think might not be available for everyone, please post an answer describing how to install it. If you are using a non-free language, but it can generate a runnable code that can be viewed with a free app, then please describe how to generate this runnable code, so other people posting in that language can follow those instructions to generate an output that can be tested by everyone.
You can also link to these language posts from your answers to help people.
One language per post please, this is a community wiki. The question is based on this meta post