A couple of archive sites from my bookmarks: First (via things magazine), the bitsavers project, an archive of software and documentation "for minicomputers and mainframes, from the 50's to the 80's." Doesn't look like the archive is being curated and catalogued at this point.
Hmm. I have some old listings in my storage unit. I wonder whether I can coax Gary and Greg to place SPREAD (a report writer for timesharing services, minicomputers—really, anything with a FORTRAN compiler) in the public domain.
Second, Karl Kleine's Historic Documents in Computer Science, a mix of archived material and links elsewhere. Some examples: The first FORTRAN manual, by John Backus and others, from 1956; Dennis Ritchie's C reference manual from about 1976. Unfortunately, the archive's last update was in October 2003, which may explain why it's missing some key material, like (ahem) anything on BASIC or COBOL.