Pencil is a standalone app that lets you create diagrams or UI mockups. It doesn't have collaboration/multi-user features.
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.
Moqups is an all-in-one online design platform that's smart, simple and fast. Go from diagrams to wireframes and prototypes without switching apps or updating across platforms.
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.
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.
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.