MockFlow!

Lucidchart is an online diagram and flowchart software that supports Microsoft Office Visio import and can work on any operating system. The easiest way to draw flowcharts, wireframes, UML diagrams, network diagrams, mind maps, iPhone mockups, site maps and more. Work together in real time with your team and clients! Along with the online version, Lucidchart also has a downloadable solution that runs on Mac, Windows and Linux; this enables offline editing at any time! »

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

Lucidchart is an online diagram and flowchart software that supports Microsoft Office Visio import and can work on any operating system. The easiest way to draw flowcharts, wireframes, UML diagrams, network diagrams, mind maps, iPhone mockups, site maps and more. Work together in real time with your team and clients! Along with the online version, Lucidchart also has a downloadable solution that runs on Mac, Windows and Linux; this enables offline editing at any time! »

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.

OmniGraffle can help you make eye-popping graphic documents—quickly—by providing powerful styling tools, keeping lines connected to shapes even when they’re moved, and magically organizing diagrams with just one click. Create flow charts, diagrams, UI and UX interactions, and more. Whether you need a quick sketch or an epic technical figure, OmniGraffle and OmniGraffle Pro keep it gorgeously understandable.

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

Diagramo is an online flowchart software

Cacoo is a user-friendly online diagramming tool that allows you to create a variety of diagrams such as wireframes, flowcharts, UML diagrams, network diagrams, mindmaps, sitemaps, database diagrams, and more.

Free web and mobile mockup tool.Transform your design and mockups into clickable interactive applications. Perfect way to validate ideas faster with lean startup thinking.

Origami Studio is a free design tool built by Facebook designers for creating interactive UI prototypes. An elegant, layer-based interface allows designers to repaidly create prototypes with layers, build interactions and then share them with others.

Principle makes it easy to create animated and interactive user interface designs. Whether you're designing the flow of a multi-screen app, or new interactions and animations, Principle lets you create designs that look and feel amazing.

UXPin is a web-based system for managing design documents like wireframes, sitemaps, persona descriptions or flowcharts. It lets you upload image files, invite teammates to review and comment on them, and keep track of revisions. The system also includes its own wireframe editor that supports real-time collaboration and allows you to export your wireframes to HTML. Another feature worth noting is that UXPin can convert paper prototypes into digital wireframes as long as they have been created using an official UXPin notepad (available for $30 on their website). You simply take a picture of the prototype and it will automatically be recognized by the UXPin app. Prices for UXPin start at $7 per month for a single-user project. »

Flinto for Mac is a comprehensive app prototyping tool that allows you to create anything from simple tap-through prototypes, to comprehensive prototypes with impressive interactions. We’ve carefully designed Flinto for Mac to minimize complexity. There’s no programming or timelines. It’s the prototyping tool designers have been waiting for.

Fluid UI is the fastest mobile app prototyping tool available. Design and test your low and high fidelity app mockups on a mobile device with ease. Simple and UX oriented, this UI tool helps unlock creativity and will save your team time and money while gathering app requirements.