Pencil is a multi-platform desktop application developed by Evolus, a group based in Vietnam. Its attractive if that it is one of the most widely used open source and free wireframe applications.
Official links are broken
iOS Wireframe (v2)
iOS GUI (v2)
ExtJS Neptune (v14)
How to add an stencil
Download a Stencil from the internet. Some official stencil examples can be found above.
Uncompress file in a folder. Go to Menu > Tools > Developer Tools > Load Developer Stencil Directory… Select .xml file in root uncompressed folder.
Go to “Manage collections…” (button on the left bar, bottom left). Click on “Install from File”. Select stencil zip file.