Radix UI
Headless React component library — accessibility + behavior, no styling. Build on top with Tailwind or your own CSS. WorkOS maintains.
Launched 2020. Components: Dialog, Dropdown, Tabs, Tooltip, Accordion, Popover. ARIA-compliant out of the box, keyboard navigation, focus management — things that take weeks to implement correctly yourself. shadcn/ui (2023) builds on Radix + Tailwind, became a phenomenon via the copy-paste model (not an npm package). Competitors: Headless UI (Tailwind Labs), Ariakit, React Aria (Adobe), Reach UI (abandoned).