CreateML Json file Annotation Format
It's not free; there is a small monthly subscription cost.
"RectLabel Lite" is a free version of "RectLabel for object detection". https://itunes.apple.com/app/id1568231336
You can draw bounding boxes and export to Create ML for free.
After looking around and wasting time for a solution to make the annotation.json for CreateML, I just went ahead and wrote a plugin for Photoshop (I've been trying to get it on the Plugin Marketplace, but there are too many hoops to jump through for a simple plugin). You can use the slice tool and the name feature to label your region, then just export. I recommend saving each file as a PSD since it retains the slice data. The tool will export as a PNG for you in the folder. Just open as many files as you want in the annotation.json file. Hope this helps! I don't really have a way to share it here, since Dropbox links and such do not work... So you will have to message me for it.