What are some alternatives?
When comparing Terrastruct and Mobile Sketchpad, you can also consider the following products
SnapUp - Wireframe faster / create amazing user interfaces
Whimsical Wireframes - Lightning fast wireframing with real-time collaboration.
Plectica - Easily diagram anything, together
Method Wireframe Kit - Find your method of being incredibly fast in prototyping.
Source Wireframe Kit - Library of 537 blocks for desktop and mobile prototyping
UserGuiding - UserGuiding - Better User Onboarding Without Coding