Many a time before we start putting down an idea to code, we first create a rough draft. When it comes to designing the UI screens, it always helps to have some mock-up references.

If you're like me who uses windows as a platform then, you are most likely to use MS Paint software to create a mockup if you are not already familiar with the mockup editors readily available these days.

Using a mockup editor saves a lot of time, it's very convenient and easy to use and it also makes the mockup look professional. There are a number of these mockup editors available these days both free and paid.

Balsamiq is one such mockup software that’s very popular and easy to use. They have a trial version on their site. However, it's paid software for most purposes.

Recently, I had to create a mockup for one of the projects I’m currently working on. I was looking for an easy-to-use mockup editor and something free to use.

I stumbled upon this wonderful editor Pencil which is free, open source and comes under the GPL license. One of the good things about this editor is that it is available for all three major OS platforms – Windows, Linux and Mac.

This tool comes bundled with quite a few nifty features out of the box.

It has built-in collections of numerous general-purpose shapes, flowchart elements, desktop/web UI shapes, and Android and iOS GUI shapes.

I like the way you can put up a wireframe for something like a wizard page with a multi-selection grid in it in no time...

Another nifty feature I find is the ability to draw flowchart-like diagrams using connectors for linking like ...

Furthermore, it also provides several export formats for your mockup ...

I've been using it for some time now and it's now an inherent part of my development toolkit. It’s convenient to use and if you are a Firefox user an extension for the same is also available :) I would also love to hear which mockup/UI prototyping tool you find indispensable!!!