Moqups!

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.

Creating flowcharts, organizational charts, business diagrams, science illustration, network diagrams, mind map and lots more. Free Examples Download.

"If they could see what Im thinking, they'd get it!" People just understand faster with pictures. It helps them see how information is connected, whether it is complete, and how a shift in one element impacts something else. Its simpler to grasp when you can see the connections and test "what if" scenarios.

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

Diagramo is an online flowchart software

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.

Instead of starting with a blank screen, SmartDraw gives you the exact template you need. Then use simple commands to add your information and SmartDraw does the rest. There's no other drawing software like it.

"Go from idea to prototype faster with Experience Design CC (Beta), the first all-in-one cross-platform tool for designing and prototyping websites and mobile apps. Test-drive the beta and watch the video to see how we’re evolving XD to bring you exciting new ways to collaborate and deliver a whole new experience in user experience." - Adobe.com

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.