There have already been some questions about some undocumented functionality in Mathematica. Such as (please add to these lists!) How can one find undocumented options or option values in ...
What is the complete list of valid Front End Tokens? Preferably, by version number.
I have lists created by other processing steps, which are basically constituted by elements like this: ...
Which packages do you know of, that have the following properties: using custom notation (for input AND output would be preferable) well coded & designed (from your subjective perspective!)