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
About Labelbox:
Labelbox is a enterprise grade and cloud based tool to easily label data for machine learning. Labelbox streamlines data labeling workflow, from micro labeling projects for quick R&D to production grade projects requiring hundreds of collaborators. It is agnostic to data type and has open source labeling frontend with already built templates for image classification & segmentation and text classification. One can label any other kind of datasets by creating a custom labeling interface with javascript API (labeling-api.js). Additional feature includes exporting data in JSON/CSV with auto generated image masks, project & team management and labeling analytics.