What are some alternatives?
When comparing Page Flows and SnapUp, you can also consider the following products
UX Archive Animated - iOS apps animated user flows
Justinmind - Define awesome experiences through wireframes & prototypes.
User Onboarding - Onboarding teardowns of popular sites/services
Balsamiq Cloud - Fast, approachable, collaborative wireframing
UI Movement - The best UI design inspiration, daily
Method Wireframe Kit - Find your method of being incredibly fast in prototyping.