I maintain mirrors of the following sites:
Documents (but not the tape images) from Al Kossow’s bitsavers.org [original site]. Bandwidth to bitsavers.org is very low, and this mirror is updated daily, so please use it.
Howard M. Harte’s S-100 Documentation Archive [original site]. In 2005-03, Howard had to remove some manuals because of disk space limitations. Those manuals are still available here.
DFWCUG Historical CPU Preservation Society’s Manuals [original site offline]. A collection of DEC manuals, mostly rather large!
Most of the manuals at HCPS were scanned by Antonio Carlini, who uploads newer scans here.