Pencil Project is a Firefox add-on that is free, open-source diagramming tool.
These days, design programs can cost a pretty penny. Luckily, there are free programs like Pencil Project that can let you learn for free.
The sketch design feature on Pencil Project allows you to create shapes and other hand-drawn sketches from scratch with relative ease. There's also a stencil generator that makes it possible for you to preserve any basic sketch mark-ups you might want to use again for future projects, as well as a few already available stencil templates to choose from.
In addition, Pencil Project can also be used as a website prototype tool. You can quickly build diagrams with it which can ultimately turn into website mockups. Just add images, text, links, or anything else you want to have on your webpage and you'll be able to see what things look like well in advance of actually coding it yourself.
Unfortunately, Pencil Project does take some time to learn how to use. Those familiar with sketching and design programs should pick it up fairly quickly, but beginners should expect a bit of a learning curve.
If you've ever wanted to learn to sketch basic designs or even see what a website might look like before actually coding it, Pencil Project is a great way to achieve that goal.