If you're building an app one of the key elements of that app is the icons you use and, of course, you always want those to look good as they help end-users navigate their way and make your app look great. You could have the best app in the world, if it looks like crap, no one will care.
One icon set often seen in BlackBerry 10 apps is Subway from Pixle which was previously offered through their site for free in PNG but other formats required a fee of $9.99. Pixle has changed that up a bit now and has begun offering the 306 icon pack optimized for mobile devices on Github for free which includes SVG, XALM, PSD, AI and EPS formats.
So if you're working on app and are looking for some great icons to use, give Subway a look. Just be sure to read through the license terms before use so you're fully aware of those.