TrueCrypt
TrueCrypt was a virtual encrypted disk software package (freeware, source-available, but not technically considered "open source" because of its use of a nonstandard license) which was mysteriously discontinued in May 2014, leading to all sorts of theories about why this happened. Users were officially requested to switch to the Microsoft disk encryption utilities, but some have developed open-source forks of TrueCrypt instead.
The official TrueCrypt sites (truecrypt.org and truecrypt.sourceforge.net) have been pretty effectively purged of all tech docs, with even the Internet Archive copies disabled, illustrating how anybody wanting to be sure of maintaining access to some particular document ought to keep their own copy of it independently of all outside institutions which might abruptly pull it or be pressured into doing so by others wanting it to be pulled.
