Dead man's switch
A dead man's switch is, in Wikipedia's words, "is a switch that is automatically operated in case the human operator becomes incapacitated" and beside being a simple but cool project to work on, it may prove useful if you for example got threatened - your friends may notice you disappeared from the face of the internet, but will not be sure what is it about and won't know which actions to take.
So here is how I coded my own using some very simple Bash and php scripts (precise details are left out to diminish the possibility of an attack): there is a cron job running at a predefined interval, every N days. IT checks for an existence of a file, if the file exists, it is deleted and I am sent a reminder to update my status. If the file does not exists, then it assumes I was incapacitated and unable to update my status in the last N days, so it releases the payload, publishing some important content on the web.
The reminder directs me to a web page where the viewer is asked for a secret password, if the correct password is entered the status is updated, the check file restored. Every operation there is logged.
For security reasons, the software does not run on any of my computers or, if that matters, on any computer in my country, so it can't be reached by any person who has physical contact with me.
The code is trivial and sloppy, with sensitive info inside, so it is not worth or sane to publish the sources, but I am sure any of my readers can reimplement it in a matter of minutes, a couple of hours at most, reading the design specifications above.
Note: if you wonder, I did not got threatened by anyone in the FLOSS world or software world nor I feel in any immediate danger, is mostly a geeky exercise.