What are some alternatives?
When comparing UsabilitySquare and ProtoPie, you can also consider the following products
UXtweak - Online user testing and usability platform
Invision - Prototyping and collaboration for design teams
UX Flow Wireframe Prototyping System - Create beautiful user flows prototypes for your next project
Framer - Bring your designs to life.
Usabilla - Collect user feedback and improve your websites, apps and emails with Usabilla's Voice of Customer Solutions.
Balsamiq - Balsamiq. Rapid, effective and fun wireframing software.