The purpose of this collection is twofold:
Personal: I am at a point in my career in which taking notes "for the sake of having a keepsake of the time I invested" takes way too much of my already limited time. This approach not only scratches the aforementioned itch, but it is, in many ways, more useful than a 500-page TeX-typed pdf.
Contributory: As I study many subjects, either as a student or a researcher, I noticed that there aren't that many sources like the present one. Most people tend to have their go-to book for formulas and quick refreshers. I am not a one-textbook-person, so here's an attempt at something different.
Find below links to various collections, created with care and purpose, on some of my favorite topics. Relevant sources (either a book I read or a class I took) will be mentioned at the beginning of each collection. These should not be used for rote learning (memorizing formulas won't make you a field theorist!), but rather for practical application. (Although if you really need to memorize some formulas, this is also a good place to find them all together).
Unless otherwise noted, the collections should not be considered "complete" or "finalized."