I just received a signed driver by Winchiphead for their USB Serial bridge CH340. Now it should be possible to use Arduino’s equipped with that without any problems under Mac OS 10.9 to mac OS 10.13 (High Sierra).
For High Sierra you must allow the driver to load in the Security & Privacy Settings you will be asked for it once you plug in a device with this serial chip, Thanks to Robin for this info.
Until it is available on the Winchiphead Website you can download the driver package here (Updated 11.01.2017):
Little hint that Altais posted in the comments if it does not work correctly. You can try copying the usbserial kext to the folder /System/Library/Extensions. Easiest way is the terminal here: sudo cp /Library/Extensions/usbserial.kext /System/Library/Extensions
If you have problems with the driver using the latest OSX High Sierra please refer to the KB Article: https://support.apple.com/kb/HT208019
Thanks to Thomas B. for this hint.
I have not tried this by myself.