Is there a free temperature monitor program that works on a 2014 Retina MBP?
It is not really an app, but there is a Widget for on your Dashboard that can display diagnostics information like the temperature. One of them I personally prefer is iStat Pro, you can find it on Softonic (the official site is offline). Hit the large green "Descargar Gratis" (Free Download) button and right-click > open the file from your downloads folder. It will be installed in your Dashboard, where it will show temperatures by default. Confirmed by me to work on the latest (late 2013) 15" rMBP.