If you are designing apps for mobile devices, it is likely you will need a good icon set to draw upon. Glyphish is a site providing a large collection of icons, many of which are free to use.
The free version includes 200 icons and requires a small attribution on your website. The pro version costs $25 and includes 400 icons, with options for larger sizes.
For more information: http://glyphish.com/
Created specifically for iPhone & iPad toolbars and tab bars, they’re also perfect for Android, websites, t-shirts, tattoos, UI design and more.