draw.io - Online diagramming application
Thunkable - Powerful but easy to use, drag-and-drop mobile app builder.
LucidChart - LucidChart is the missing link in online productivity suites. LucidChart allows users to create, collaborate on, and publish attractive flowcharts and other diagrams from a web browser.
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.
PlantUML - PlantUML is an open-source tool that uses simple textual descriptions to draw UML diagrams.
Bubble.io - Building tech is slow and expensive. Bubble is the most powerful no-code platform for creating digital products.