FET is free, open-source timetabling software authored by Liviu Lalescu and Volker Dirr.
FET for Mac OS can be compiled from the source code available on the office website (see the instructions here), or the compiled app can be downloaded from this page. New versions will be added as they become available.
When you run FET for the first time, you will get a notice that FET is from an unknown developer (as I’m not a registered developer). In order to bypass this warning, control-click on the FET app and select “open”. (This step is only necessary the first time you run the app after downloading.)
Please report any issues you may have with the Mac version in the FET forums here.