Visual Feedback, Website Annotation & Bug Reporting Tool | Volley
Zeplin
Website Planning and Organization Tool » Jumpchart
Online Diagram & Flowchart Software - Collaboration Tool | LucidChart
iPhone Mockup
Cacoo - Create diagrams online Real time collaboration
Wireframe Software - Free website wireframe software by Gliffy
iPlotz: wireframing, mockups and prototyping for websites and applications
MockFlow: Online Wireframe Tool
Penpot - Design Freedom for Teams
Bounce A fun and easy way to share ideas on a website
Usabilla - Micro usability tests