stdint.h
It's always been amazing to me how many different projects I've worked on (not that I've been in professional C for about 7 years now)) that include their own painstaking recreation of this file.
Reusing them and effectively translating them just to your own name is just annoying to the reader IMHO. I am reminded of a C++ project I worked on, where I questioned the extensive use of typedefs around collections of things, various forms of references and compound objects etc. I was informed by one of the more experienced C++ folks that it made the code easier to comprehend.
Later I saw the typedef cheat-sheet sellotaped to the side of his monitor...