Many times, a given seffer has a commentary and or index written on it, but due to lack of knowledge about it or lack of accessibility, the commentary/index remains largely unknown.
This question requests the posting of such commentary/indexes.
The preferred format should be to name the original seffer and author in Hebrew and English, followed by the name of the commentary/index and author in Hebrew and English, along with any helpful information such as the ISBN if applicable, links etc. Please indicate if it is a commentary, index, or both.
In order to keep this from being ridiculously broad, let's limit the scope.
An extremely popular work with many super-commentaries should not be included, even if a particular commentary is completely unknown. So, for example, no commentaries/indexes on the Talmud or Rambam need apply.
The super-commentary index should be dedicated to a particular seffer, not a collation of previous works with commentary upon them. So, for example, Mishna Berurah on halacha and Sha'arei Aharon on Torah are disqualified.
Works that are considered standard editions of a given seffer, even if they include commentary/indexes. This one may be hard to quantify, but this limitation seems necessary. For example a Mossad HaRav Kook edition of a given seffer.
A set which was reprinted or added to, which now includes a commentary/index. For example, The Collected writings of Rabbi Samson Raphael Hirsch. The ninth volume includes an index for the entire set, but that ninth volume was only put out years later. There are people who own and use the original set, but are unaware of the index in the additional volume.
Another example, The fifth volume of Michtov Me'Eliyahu, which has an index, was not included in the set originally, afaik. I'm not 100% sure about this one. Let's see what the comments bring, I can update the question based on community response.