We've also been working on a similar tool (although does not output to YOLO format): https://www.labelbox.io/
We've open sourced the labeling interface for anyone to easily build and support labeling any data - as long as it can be loaded in a browser. Learn more here: https://github.com/Labelbox/Labelbox
> "You only look once (YOLO) is a state-of-the-art, real-time object detection system."
I'd also include the "image classifier" keyword.
On a side note: I am working on a client side annotation tool which is backed by OpenCV and Mask R-CNN. It is still in the early stages (and may contain some bugs), but in case you are interested, you can check it out here 
The whole service is running on a really small instance at the moment (will be moved soon), so it's rather slow at the moment.
We've been working on a platform for medical image and video annotation tasks. This means we need to support everything from DICOM to large pathology images, and endoscopy videos. We also have it connected to deep learning networks (e.g. Inception v3, YOLO, ENet) so you can easily train or download the JSON for offline analysis.
We'd love to have beta testers from the HN community:
Drawing bounding box, polygon, and cubic bezier
1-click buttons make your labeling work faster
Customize the label dialog to combine with attributes
Settings for objects, attributes, hotkeys, and labeling fast
Layer order for overlapped boxes
Zoom in on a point
Quick zoom to existing boxes
Support the PASCAL VOC format
I wanted it to give automatic suggestions.