What are some alternatives?
When comparing Page Flows and Whimsical Wireframes, you can also consider the following products
UX Archive Animated - iOS apps animated user flows
Method Wireframe Kit - Find your method of being incredibly fast in prototyping.
User Onboarding - Onboarding teardowns of popular sites/services
Mydraft.cc - 100% free wireframing
UI Movement - The best UI design inspiration, daily
UXpin - Design is really about solving problems. UXPin is the UX Design Platform that gets that right.