quickMockup!

Pencil is a standalone app or Firefox extension that lets you create diagrams or UI mockups. It doesn't have collaboration/multi-user features.

Using Balsamiq Mockups feels like you are drawing, but its digital, so you can tweak and rearrange controls easily, and the end result is much cleaner. Teams can come up with a design and iterate over it in real-time in the course of a meeting.

InVision's prototype tool lets you create interactive wireframes and high-fidelity prototypes. Share your work and get instant feedback right on-screen.

Axure RP is the leading tool for rapidly creating wireframes, prototypes and specifications for applications and web sites.

Mockup Designer, built with Backbone, Backbone-LocalStorage and Html2Canvas, is a super-easy-to-use mockup tool that uses the local storage of your browser as its backend. However the data layer is loosely coupled, so you can integrate with any RESTful API or any Backend service.

WireframeSketcher is a software tool that helps quickly create wireframes, mockups and prototypes for desktop, web and mobile applications. It comes both as a plug-in for any Eclipse-based IDE and a standalone version. With WireframeSketcher users can quickly gather useful feedback from stakeholders, show off their proposals to clients, share their ideas with fellow developers and in the end build better software.

A functional wireframe is the best way to reduce costs of your software projects. Simulate your wireframes and mitigate risks.

Build your idea into a product.Design and programming skills are no longer necessary to create a product. With STUDIO, you can make your idea into a real product regardless of your skill and knowledge.

mockingbird is an online tool that makes it easy for you to create, link together, preview, and share mockups of your website or application.It is not Flash based.

Design your software or website mockups within minutes. Mockup Builder is a new look to solve old problem with prototyping and sharing sketches with customers. This startup was built to solve developer’s needs in product that can help to make it better communication tool between developers and clients in site developing process.

HotGloo is a UX, wireframe and prototyping tool designed to build wireframes for web, mobile and wearables. HotGloo helps to visualize planning processes, build and test drive interactions very easily. Create and share fully interactive prototypes with your team and clients and gather feedback on the process. From prototypes of the next award winning app to huge e-commerce wireframe shop solutions, HotGloo is the perfect match for every web worker.

MockupCreator is a tool to create mockups, prototypes and wireframes.

Quickly create, share and simulate professional wireframes, mock-ups and UI prototypes online without programming. Pidoco is an easy-to-use web-based prototyping application that comes with real-time collaboration and usability testing features and can speed up your projects by over 50%!