Indigo Studio is a free tool for quickly creating prototypes of web, mobile or desktop apps.

The application works on both Windows + Mac OS X and not only capable of generating static outputs but also supports interactions (links, animations, etc.).

Usually, prototypes can look plain and customers can have difficulty in imagining how things would look in real life. Indigo Studio solves this by providing lots of storyboard scenes with people, places and various devices to create a more realistic look.

It is also possible to import hand-drawn sketches into the application and adding interactivity to them.

Indigo Studio is a pro quality yet free application and only requires a free membership to the site.

There are thousands of design freebies and each of us are probably using them regularly.

When it comes to finding such freebies, Freepik, a search engine that indexes only vectors, PSDs and photos can be very useful.

The search engine crawls many design-related websites, ranks the found resources with their relevance and presents to the user.

It is possible to narrow the search with the file type (vector, photo, PSD) and categories offered (nature, buttons, icons, background, etc.).

Also, the website offer various premium design materials like stock videos, images and design deals.

When creating wireframes offline, papers with grids help a lot to create outputs that look better.

Dotted Paper is just a free PDF file which makes wireframing/designing easier with the horizontal and vertical grid it has.

Although that is a simple resource and can be created with not much effort, it is very handy as it is ready to download : ).

Radi is a complete visual design application targeting web with 100% standard and human-readable HTML + JavaScript code.

It has built-in tools for creating/editing graphics (they can be imported too) and generating animations with <canvas> output that work in all modern browsers including mobile.

There are ready-to-use filters + effects and you can always add your own code to create more customized stuff.

The application is also pretty powerful in rendering video. It can even -very quickly- convert a web animation into a HTML5 video.

Radi has a very short learning curve as it is like a mixture of well-known image editing and animation tools but with the friendliness of web technologies.

While working on responsive websites, we quickly want to see how the layout reacts to different screen sizes.

Viewport Resizer is a very useful bookmarklet for responsive-layout-testing with few clicks.

Once clicked, it shows a list of popular screen sizes and we can see the websites in those sizes by simply pressing them.

Its page allows us to create a customized bookmarklet that includes the sizes we want.

The orientations of various sizes are already set but there is also an option to manually change the orientation.

There is support for media queries and it is touch-friendly.

In the past, "few resources for sketching websites on paper with ease" was shared at WRD.

Here is a new one -Responsive Sketchsheets- that fulfills the need of sketching responsive website layouts by ZURB.

The download pack includes a single PDF file with multiple scenarios: "only mobile", "mobile and desktop", "tablet (with orientation)".

Also, another package is provided which includes the standard sketchsheets by the same company.

When designing websites and/or apps, sketching ideas can help thinking on ideas visually and ease discussing on them.

Interface Sketch is a pack of free browser sketch sheets for web, mobile and tablet.

They are available as PDF files, provided in 2 sizes: A4 + US letter and includes the most popular devices like iPad, iPhone 4-5, Nexus Tablet, Android Phone or Windows 8.

There are various solutions exist for compressing and minimizing the size of images (like Smush.it, PNGGauntlet, punypng, Kraken.io and more).

Here is another good one: TinyPNG that only works with PNG files, shrinks them -nearly- without loss that an eye can see and by keeping the transparency.

It works by drag 'n' drop uploading the images to be minimized to the web interface and auto-generates the compressed ones.

Using TinyPNG, it is easily possible to get 50%+ smaller sized images as they are converted to 8-bit PNGs (rather than 24bit) by using smart techniques.

Smashing Magazine has released a very handy freebie for designers/developers that are working on mobile project.

It is a set of UX sketching and wireframing templates which consists of 2 parts:

Outline

This is a pack of 28 printable sketching and wireframing papers (in PDF) for the most popular mobile platforms.

There are various combinations in the set like actual size, 10 devices fit to a page, and landscape layout.

Also, an Illustrator file which displays the mobile devices is provided.

Tapsize

These are papers for easily analyzing the optimal tap areas without playing with a real mobile device.

It has 9 PDF files that come in different sizes.

Creating wireframes in the beginning of any design process can help creating an output without any surprises.

Sneakpeekit is a website providing high-quality PDF sketch sheets for web designers.

The sheets are compatible with the most popular grid systems like Less Framework 4, 978 grid system, 1140 css grid, The Semantic grid system, Bootstrap from Twitter, etc.

And, they are not only for the desktop browsers but there are versions for tablets and mobile as well.