Top Pencil Project Alternatives for Diagramming and UI Mockups
Pencil Project is a free and open-source tool for creating diagrams and GUI prototypes. Available as a standalone application or a Firefox extension, it offers built-in stencils, multi-page documents, inter-page linking, and various export options. However, for users seeking more advanced features, real-time collaboration, or broader platform support, exploring a Pencil Project alternative is often necessary.
Best Pencil Project Alternatives
Whether you're looking for cloud-based collaboration, advanced vector graphics, or specialized UI design tools, there's a strong Pencil Project alternative out there for your needs. Here are some of the top contenders:

diagrams.net
diagrams.net (previously draw.io) is a powerful, free, and open-source online diagramming application available on the web and for self-hosting. It's an excellent Pencil Project alternative for workflow, BPM, org charts, UML, ER, and network diagrams, offering features like export to PDF, SVG, Visio stencil support, and integration with Google Drive and Dropbox.

LibreOffice - Draw
LibreOffice - Draw is a robust, free, and open-source vector graphics editor part of the LibreOffice suite. Available for Mac, Windows, Linux, and BSD, it's a strong Pencil Project alternative for easily creating charts, diagrams, and brochures, with features like sequence diagrams and PDF export.

Sketch
Sketch is a commercial design toolkit exclusively for Mac, ideal for UI design from initial ideas to final artwork. As a Pencil Project alternative, it offers advanced features like live preview, cloud sync, bitmap editing, integration with InVision, screen mirroring, and powerful vector drawing capabilities.

Figma
Figma is a popular freemium collaborative interface design tool available across Windows, Linux, Android, iPhone, and iPad, as well as a SaaS solution. It's a fantastic Pencil Project alternative for teams needing real-time collaboration, prototyping, cloud sync, UI design, and features like importing from Sketch and responsive design.

Microsoft Office Visio
Microsoft Office Visio is a commercial diagramming and flowchart software, part of the Microsoft Office Suite, available for Windows. For users within the Microsoft ecosystem, it serves as a comprehensive Pencil Project alternative for creating complex diagrams, flowcharts, network maps, and workflows.

Adobe XD
Adobe XD (Experience Design) is a freemium vector-based tool from Adobe Inc. for designing and prototyping user experiences for web and mobile apps. Available on Mac, Windows, Android, iPhone, and iPad, it's a powerful Pencil Project alternative offering interactive mockups, responsive design, UX design, animation, and live preview capabilities.

Balsamiq Wireframes
Balsamiq Wireframes is a commercial, low-fidelity UI wireframing tool available on Mac, Windows, and Web. It's an excellent Pencil Project alternative that excels at reproducing the experience of sketching on a notepad, offering quick drawing and real-time collaboration for wireframing.

OmniGraffle
OmniGraffle is a commercial graphic documentation tool available for Mac and iPad. It's a strong Pencil Project alternative for creating eye-popping graphic documents, offering powerful styling tools, connected lines, and capabilities for flowcharts, ER diagrams, graphs, and workflows.

Scapple
Scapple is a commercial tool for Mac and Windows designed for quickly getting ideas down and making connections. While not strictly a diagramming tool, it's a valuable Pencil Project alternative for visual brainstorming and notetaking, offering a free-form canvas and fullscreen support.

Apache OpenOffice Draw
Apache OpenOffice Draw, part of the Apache OpenOffice suite, is a free and open-source graphics editor available for Mac, Windows, and Linux. As a Pencil Project alternative, it provides comprehensive tools for visual communication through diagrams and other visuals, including desktop publishing and vector drawing.
Each of these alternatives offers unique strengths, from real-time collaboration and advanced UI/UX features to robust diagramming and vector graphics capabilities. We encourage you to explore these options further to find the best fit for your specific project needs and workflow.