20th century advertising has taught us to associate quality artwork and polish with quality products. Given two apps that do the same thing, a potential customer will pick the one that looks and feels better. This means every great app needs great art. Since most developers aren't artists or designers by trade, I've assembled a list of resources that can help. Here are icons, fonts, sounds, color schemes, and other great art assets to help you make your app stand out from the crowd.

Color Schemes

Good use of color can really make your app stand out, but color can be tricky. The best color schemes often come from other people or real world objects. These sites have collections of color schemes created by people who work with color every day. They let you search by color, theme, and popularity.

COLOURlovers

COLOURlovers is a creative community where people from around the world create and share colors, palettes and patterns, discuss the latest trends and explore colorful articles... All in the spirit of love. (yes, they wrote that part)

FirefoxScreenSnapz023.png

Kuler

Kuler at Adobe is a Flash based color searching site. FirefoxScreenSnapz022.png

For more on the topic of color see my Color 101 article.

Icons

Every great app needs great icons. Not only for the app itself but also within the application for buttons and indicators.

The great FXExperience blog has several free PNG icon collections that are generic and useful for lots of things. Most are small so they work well on mobile devices. Free icons for your JavaFX applications @ FX Experience

The Crystal Clear icon set by Everaldo is licensed under LGPL and includes icons for apps, actions, devices and mimetypes.

The Nuvola icon set contains 600 icons with a cute cartoon feel to them.

Icon sets for Gnome, under various licenses

Tango Icon Library: another very complete set of desktop icons. FirefoxScreenSnapz021.png

Sounds

FreeSound.org

Free Sound has a huge collection of user contributed effects, clips, and just plain weird sounds. Great for building sampled music and effects in your games

FirefoxScreenSnapz025.png

Jamendo

Jamendo is a music site containing only Creative Commons licensed works. The perfect place to find your next soundtrack.

PreviewScreenSnapz001.png

Imagery & Textures

Lost Garden

Lost Garden focuses on video game design. The author, Danc, has literally decades of experience in the field. You can easily lose hours reading through amazing essays on the site. Today, however, we are here for the free game graphics. Spanning both vector and bitmap, retro and modern styles, Lost Garden has game graphics for many uses

Open Graphic Design and Think Design blog

OpenGraphicDesign.com has tons of cool shapes and vector artwork. They are great as starting points in Illustrator

ThinkDesignBlog.com has free textures and vector shapes. Good for backgrounds and skinning. Plus tons of inspiration articles.

Photos

For photos your best source is images licensed under Creative Commons at Flickr. Conveniently they have a search option for just such photos.

Inspiration

Well Placed Pixels is a blog containing only one thing: screenshots of beautiful software.