What are some alternatives?
When comparing hireframes.com Mobile Sketchpad and Whimsical Wireframes, you can also consider the following products
Source Wireframe Kit - Library of 537 blocks for desktop and mobile prototyping
Method Wireframe Kit - Find your method of being incredibly fast in prototyping.
Mydraft.cc - 100% free wireframing
Contra Wireframe Kit - Open source wireframe kit for design and prototyping 🚀
UX Flow Wireframe Prototyping System - Create beautiful user flows prototypes for your next project
Wireframe Deck of Cards - A physical way to build and revise website and app layouts