Label Studio

Label Studio is the open-source Swiss Army knife for data labeling — images, text, audio, video, time-series, and HTML, all annotated through a single customizable web UI. If you're training a model and need labeled data without paying Labelbox $1,000+/month, start here.

For indie hackers building ML products with small-to-medium datasets, Label Studio is the free, flexible option. The labeling interface is configurable via XML templates. CVAT is the alternative for pure computer vision annotation. Labelbox and Scale AI are the enterprise options with automation and workforce management. Encord is the mid-tier commercial player.

The catch: "Free and flexible" means you're the QA department. Label Studio doesn't auto-label or prioritize — it's a manual annotation tool that you need to organize around. The enterprise edition (by HumanSignal) adds ML-assisted labeling and team management but costs money. Performance degrades on very large datasets. For serious ML training at scale, you'll outgrow the open-source version.