Fortran Standards Documents
Draft (unofficial) versions of the various recent Fortran standards and corrigenda are available from the J3 website, the WG5 website, and/or the WG5 file server, in various formats. In addition, some older Fortran standards are available from www.fortran.com. The official standards can be bought from the International Organization for Standardization (ISO) or from your national standardization organization.
Note that, of these standards, GNU Fortran currently only supports the base Fortran 95 standard (not parts 2 or 3) as amended in the two corrigenda, the Fortran 95 TR 15581 extension, the Fortran 77 standard with the MIL-STD 1753 extensions, and some portions of the Fortran 2003 standard; these standards have been highlighted in bold. The other documents are linked here for reference, but should not be taken as documentation of GNU Fortran features.
Fortran - Automatic Coding System for the IBM 704
The oldest version; dated October 15, 1956, first presented in February 1957 and first delivered in April 1957. Fortran - Programmer's Reference Manual (PDF)
DoD Extensions to Fortran 77 (MIL-STD 1753): (html)
Fortran 90 Standard ISO/IEC 1539:1991
(ISO Varying strings are not implemented in GNU Fortran, but Fortran 95 module exists which implements this is Fortran.)
(Conditional Compilation is not supported directly by GNU Fortran but you can preprocess the files using COCO.)
Fortran 95 Corrigendum 1, draft: (pdf)
Fortran 95 Corrigendum 2, draft: (pdf)
Other Relevant Standards
The IEEE standard covering Fortran 77 POSIX bindings is available online, though unfortunately only from locations with appropriate subscriptions to the IEEE server (e.g., many university networks). For those who do have such access, the link is:
POSIX Fortran 77 Language Interfaces (IEEE Std 1003.9-1992) (pdf)