Kanboard - Kanboard is a free and open source Kanban project management software.
Thunkable - Powerful but easy to use, drag-and-drop mobile app builder.
Excalidraw - Excalidraw is a whiteboard tool that lets you easily sketch diagrams that have a hand-drawn feel to them.
Xamarin.Android - Integrated environment for building not only native Android but iOS and Windows apps too.
Kinopio - Thinking canvas for mind mapping, brainstorming, and moodboards
Kodular - Much more than a modern app creator without coding