Small amount of bash shell code to produce terminal output. I use this to keep alive ssh connections where the server will automatically disconnect a session where input is not received for a small amount of time.
Needs: A terminal that understands ansi.sys escape codes. Most modern interactive terminals. You might have limited success on serial terminals.
CAVEAT: Admittedly

watch date

would do the same job - but is less fun.
In use I have this in a function in my .bashrc

function keepalive () {

}