Duong Thanh An, has just released Pencil, a simple and efficient open source tool to create user interface prototypes with native Windows and Linux GTK widgets.
Using Pencil is pretty straight forward. You start with a new document and just start adding widgetsfrom the toolbar to the canvas. It includes a variety of basic elements like windows, menus, tables, progress bars, checkboxes, buttons, radio buttons, etc. as well as basic annotation tools like text, images, and arrows, to add comments.
You can also add images but be aware that by default Pencil will just link to the file instead of embedding the image data in the document, so if you delete the source it will be gone from the prototype as well. You can embed the actual image by pasting it but Duong warns thatit may affect performance.
A document can have multiple pages each with a different prototype so you a document can hold a full project. Objects can be easily aligned, organized, grouped and locked to prevent accidentally moving or modifying a finished one.
As a first release there are things missing, like more annotation elements including more arrow types and shapes, an option for adding comments as object properties instead of a visual element. Some widgets like tables and combo boxes could get a little smarter and allow entering multiple values instead of adding a lot of text boxes. Mac OS X and Windows Vista native widgets would also be a big a plus for a next version as well as an option to organize the document pages in folders or at least an option to tag and search them.
For some reason Pencil made accessing advanced preference (though about:config in the location bar) generate an error. It could be related to an Advanced Settings… item in Pencil’s menu that launches the advanced preferences. Disabling Pencil brought about:config back.
Anyway, it felt pretty solid specially for a first release and looks like a keeper for me and those times I need to make a mockup.
Pencil is available as a 390 KB Firefox 3 extension or an 8MB XULRunner based stand-alone application for Windows and Linux only at this time. You can get it from the Pencil project web site.