Appcelerator Titanium - Appcelerator Open Source
Thunkable - Powerful but easy to use, drag-and-drop mobile app builder.
React Native - A framework for building native apps with React
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.
Onsen UI - HTML5 Hybrid Mobile App UI Framework - work with Angular, React, Vue, Meteor & pure JavaScript. Material & Flat design.
Bubble.io - Building tech is slow and expensive. Bubble is the most powerful no-code platform for creating digital products.