Thunkable - Powerful but easy to use, drag-and-drop mobile app builder.
placeholder.pics - Lightest way to include placeholder images in your design
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.
Fill Murray - Custom-sized placeholder images of Bill Murray
Bubble.io - Building tech is slow and expensive. Bubble is the most powerful no-code platform for creating digital products.
PlaceKitten - Dead-easy placeholder images. Of kittens.