Pencil is a standalone app that lets you create diagrams or UI mockups. It doesn't have collaboration/multi-user features.Discontinued
The last release/commit was in October 2019.
Balsamiq Wireframes is a rapid low-fidelity UI wireframing tool that reproduces the experience of sketching on a notepad or whiteboard, but using a computer.
Wireflow is free, online, real-time collaborative, open source tool for creating beautiful user flow prototypes.
We believe the screen is the most important place in the world. That’s why we are dedicated to helping you deliver the best possible digital product experience, with our platform and best practices from your peers.
With the Marvel design platform, access all the core functionality you need to build digital products - wireframe, prototype and generate design specs in one place.
POP helps you make interactive prototype with ease. If you can draw, you can design apps. The workflow is ridiculously simple:This app doesn't have any features at the moment.
STUDIO lets you easily design & publish websites from scratch without writing any code. With the option of starting from a blank canvas, wireframe or template, you are free to unleash your creativity.
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.