Balsamiq Mockups!

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

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.

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.

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. »

"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

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.

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.

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.

A free tool for rapid prototyping, fun wireframing and quick mockups.Create mobile app wireframes for iOS, Android, Windows Phone, Surface, design website mockups, share, collaborate and test usability for free!

Nothing to download/install, create and access your mockups from anywhere.

If you are looking for a tool that makes you design easier and faster, you cannot go pass Mockplus.1. Fast Interaction- Interactive design is easier than ever

Designed specifically for mobile, Proto.io can simulate everything an app can do. That includes interactive touch gestures, screen transitions and animating any UI item in a prototype. It is perhaps the only mobile prototyping tool that allows users to create realistic, sharable prototypes that work as a real app should and experience their prototype on the actual device. Proto.io is also the first prototyping tool to ever include custom device support, to allow prototyping for anything with a screen interface - from SmartTVs to refrigerators. »

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.