Overflow.io is described as 'The world’s first flow diagramming tool tailored for designers. Build and present beautiful user flow diagrams that tell a story. Request early beta access'. There are 2 alternatives to Overflow.io, not only websites but also apps for SaaS and Marvel. The best alternative is Userflows, which is free.
Userflows turns your Marvel prototypes into interactive user journeys that give you a birds-eye view of how users will move from screen to screen, helping you tell a better story to developers, stakeholders and clients.
Primary helps teams to create better products, faster. Visualise, document, validate and handoff product design. Primary provides a structured approach so users and developers can work together to establish what jobs your app will do and how it will do them.