Pencil is a tool created with Mozilla Gecko engine, which is used for creating mockup screens for designing prototypes.
This is an opensource tool and works on any platform that Firefox 3 can.
Several built-in widgets(stencils) are available which can be individually customized and reused as required.
Web designers can collaborate their ideas with the help of built-in annotation stencils.
The pages created can be exported in PNG format for sharing or uploading the screens.
TIP: Create a template page and do all the customizations in it. Once you are satisfied with the layout, use the duplicate option to reuse the template to make all the subsequent pages are consistent.
Features:
- Built-in stencils for diagraming and prototyping
- Multi-page document with background page
- On-screen text editing with rich-text supports
- PNG rasterizing
- Undo/redo supports
- Installing user-defined stencils
- Standard drawing operations: aligning, z-ordering, scaling, rotating…
- Cross-platforms
- Adding external objects
Pencil is available in standalone portable mode for linux and windows OS (Size : ~ 9MB) and as a firefox addon (~400KB)
Download Pencil Mockup : 
Go to Pencil Project Home
