The browser-based design tool where interfaces are drawn, components are maintained, and prototypes are assembled. Everyone works in the same file simultaneously and comments directly on the design.
Design review stops being a cycle of exported images and email threads. Stakeholders see the current state at a permanent link, and developers read exact values instead of measuring screenshots.
We keep one source file per product with a component library and tokens, hand over with specs developers can read directly, and keep the file updated after launch so it never diverges from the live product.