What are some alternatives?
When comparing Mobile Sketchpad and Proto.io, you can also consider the following products
Whimsical Wireframes - Lightning fast wireframing with real-time collaboration.
Invision - Prototyping and collaboration for design teams
Method Wireframe Kit - Find your method of being incredibly fast in prototyping.
Balsamiq - Balsamiq. Rapid, effective and fun wireframing software.
Source Wireframe Kit - Library of 537 blocks for desktop and mobile prototyping
Moqups - The most stunning HTML5 app for creating resolution-independent SVG mockups, wireframes & interactive prototypes for your next project