Not quite free, but rail passengers holding the correct ticket can use route 398 between Glasgow Central and Glasgow Queen Street free of charge.
Also, we had one in fife until last week. Due to a car park being closed for the building of a enw Tesco store in Dunfermline city centre, Tesco subsidised a free shuttle bus from Leys Road car park (built on the former approaches to the long gone Dunfermline Upper train station), and Dunfermline bus station. The new Tesco is open, and so the shuttle, run by Stagecoach East Scotland, has ended.