Label Studio

Draw bounding boxes on images, highlight entities in text, transcribe audio, or create custom labeling interfaces with a template system. It's the Swiss Army knife of data labeling.

The community edition is free under Apache 2.0. You get multi-user support, a project system, customizable labeling interfaces, and import/export in every format (COCO, YOLO, spaCy, etc.). Self-host it with Docker and you're labeling data in 10 minutes.

The catch: the free version is single-node only. Label Studio Enterprise (now called HumanSignal) adds team management, active learning, model-assisted labeling, RBAC, SSO, and analytics, but pricing requires a sales call. The community edition handles small-to-medium labeling projects well, but once you have 5+ annotators who need quality control and agreement metrics, you'll feel the feature gap.