InVision is a prototyping, collaboration and workflow tool that offers cloud-based solution to design and wireframe mobile apps and desktop sites. You can install on popular operating systems such as Mac OSX, Linux Ubuntu, and Windows 32 and 64 bit.
Pencil is a free open-source GUI prototyping desktop software that can easily install and use to create mockups. It offers hundreds of free templates and shapes with easy drag and drop functionality to create professional flowcharts, process maps, UML models, org charts, wireframing, iOS mockups and more. Lucidchart free version is limited to one user and 60 project objects. Lucidchart is a free wireframe builder and UI design tool which is a solution for visual communication and cross-platform collaboration. You can invite your co-worker and together in real-time design the user flow for your project. Having over 100 custom build graphics and cards to use web elements. Wireflow comes with collaboration in mind. Wireflow is free online and open source wireframe tool for creating beautiful user flow prototypes.
