What is some (useful) piece of software that is written to be as simple as possible?
I'll cite some:
>slock
alternative to xlock:
http://tools.suckless.org/slock/
>st
alternative to xterm:
http://st.suckless.org
>xclip
clipboard from command line:
https://github.com/astrand/xclip
>keynav
mouse from keyboard:
http://semicomplete.com/projects/keynav/
>lel
image viewer:
http://git.2f30.org/lel