Archives of old technical magazines and journals. Mirrors of particularly important retrotech archives.
Mirror friendly as long as you use anonymous rsync. Please note that their archive is rapidly approaching 1TB in size, so you may wish to think carefully about what you want and why.
A summary of how to set up a full Wikipedia.org mirror using several different approaches: Nginx as a caching proxy in front of wikipedia.org, Kiwix serving a downloaded backup, a full Mediawiki install with a database dump, or a mirror of Wikipedia made with XOWA.
OpenStreetmap's wiki page pertaining to planet.osm, the planet-wide database dump of the OSM. Includes formats, mirrors, bittorrent, and technical notes. Might be useful to read.