SystemV x86-64 ABI (official standard). Used by all OSes but Windows. (This github wiki page, kept up to date by H.J. Lu, has links to 32bit, 64bit, and x32. Also links to the official forum for ABI maintainers/contributors.) Also note that clang/gcc sign/zero extend narrow args to 32bit, even though the ABI as written doesn't require it. Clang-generated code depends on it.
OS X 32bit x86 calling convention, with links to the others. The 64bit calling convention is System V. Apple's site just links to a FreeBSD pdf for that.
__fastcall calling convention
__vectorcall: documents the 32bit and 64bit versions
__stdcall: used used to call Win32 API functions. That page links to the other calling convention docs (e.g.
Why does Windows64 use a different calling convention from all other OSes on x86-64?: some interesting history, esp. for the SysV ABI where the mailing list archives are public and go back before AMD's release of first silicon.