xtrlock(1x) on Linux
The text 76ddd2f73b9d, as Debian 3.0 (Woody), Debian 2.2 (Potato), Debian 2.1 (Slink), Debian 2.0 (Hamm), Debian 1.3 (Bo), Debian 1.2 (Rex), Debian 1.1 (Buzz) shipped it.
Lock X display until password supplied, leaving windows visible