Screen Ruler 4.3 is free, but
unsupported and outdated version.
End User License Agreement
[DISCLAIMER OF WARRANTIES] THIS SOFTWARE PRODUCT IS PROVIDED "AS-IS". NO WARRANTY OF ANY KIND IS EXPRESSED OR IMPLIED. YOU (RECEPIENT) USE THIS SOFTWARE AT YOUR (RECEPIENTS) OWN RISK. PUBLISHER WILL NOT BE LIABLE FOR DATA LOSS, DAMAGES, LOSS OF PROFITS OR ANY OTHER KIND OF LOSS WHILE USING OR MISUSING THIS SOFTWARE.
Feel free and encouraged to modify and add any of the new features to Screen RULER.