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.
Figma - Team-based interface design, Figma lets you collaborate on designs in real time.
VisualSitemaps - Autogenerate visual sitemaps in seconds.
Invision - Prototyping and collaboration for design teams
W3C Markup Validation Service - Validates HTML markup of web pages, one at a time.
Sketch - Professional digital design for Mac.