Thunkable - Powerful but easy to use, drag-and-drop mobile app builder.
Source Wireframe Kit - Library of 537 blocks for desktop and mobile prototyping
MIT App Inventor - App Inventor is a cloud-based tool, which means you can create apps for phones or tablets right in your web browser.
Platforma - Wireframe library of layouts and UI elements
Bubble.io - Building tech is slow and expensive. Bubble is the most powerful no-code platform for creating digital products.
Contra Wireframe Kit - Open source wireframe kit for design and prototyping 🚀