Octopus.do - Build your website structure in real-time and rapidly share it to collaborate with your team or clients. Start prototyping websites or apps instantly.
Invision - Prototyping and collaboration for design teams
PowerMapper - Create visual HTML site maps and XML sitemaps for search engines of any web site quickly and easily.
Zeplin - Collaboration app for UI designers & frontend developers
FlowMapp - FlowMapp is a UX planning tool for creating visual sitemaps and user flow.
Balsamiq - Balsamiq. Rapid, effective and fun wireframing software.