Storybook

Storybook gives you a dedicated workshop for developing and testing UI components (buttons, forms, cards, modals) in isolation. You write 'stories' that render your components in different states, and Storybook serves them in a browsable catalog. It's a workbench where you see every component in every possible state.

Everything is free under MIT. The core tool, all framework integrations (React, Vue, Angular, Svelte, Web Components), addons for accessibility testing, viewport switching, and interaction testing, all open source. Chromatic (their hosted visual testing service) is a separate paid product.

The catch: Storybook adds build complexity. It's another build tool, another config file, another thing that can break when you upgrade your framework. For small projects with 10 components, the overhead isn't worth it. It shines when your design system has 50+ components and multiple developers need to understand what exists and how it works.