What are some alternatives?
When comparing Mobile Sketchpad and Whimsical Wireframes, you can also consider the following products
Method Wireframe Kit - Find your method of being incredibly fast in prototyping.
Source Wireframe Kit - Library of 537 blocks for desktop and mobile prototyping
Mydraft.cc - 100% free wireframing
Wireframe.cc - Sweet and simple online wireframing tool.
UXpin - Design is really about solving problems. UXPin is the UX Design Platform that gets that right.
Contra Wireframe Kit - Open source wireframe kit for design and prototyping 🚀