xtrlock locks the X server till the user enters their password at the
While xtrlock is running, the mouse and keyboard are grabbed and the
mouse cursor becomes a padlock. Output displayed by X programs, and
windows put up by new X clients, continue to be visible, and any new
output is displayed normally.
The mouse and keyboard are returned when the user types their
password, followed by Enter or Newline. If an incorrect password is
entered the bell is sounded. Pressing Backspace or Delete erases
one character of a password partially typed; pressing Escape or Clear
clears anything that has been entered.
If too many attempts are made in too short a time further keystrokes
generate bells and are otherwise ignored until a timeout has
The X server screen saver continues to operate normally; if it
comes into operation the display may be restored by the usual means of
touching a key (Shift, for example) or the mouse.
On my Debian system, it's in the repositories:
sudo aptitude install xtrlock