slock - simple screen locker
The most simple screen locker utility available for X.
Edit config.mk to match your local setup (slock is installed into the /usr/local namespace by default).
In order to build and install slock execute
sudo make clean install
Lock X by displaying a black screen and waiting for your input.
Lock X whereas your screen looks unlocked. If a mouse button is hit the screen becomes black and a snapshot of the unwanted user is taken and rendered. In order to use this spy mode you need to have fswebcam installed.
Just enter your password.