#keynotopia

Sketch

Sketch is like Photoshop, only leaner. This means that the learning curve is much shorter, and you can easily start designing and exporting mockups in no time. One of the great features of Sketch is that you can export designs that are optimized for all platforms at once, which means you don't have to build out multiple versions of each of your objects for various devices (iOS, desktop, etc.) - creating responsive designs has never been easier. Did we mention its super affordable? With a $99 price tag, this piece of software is a must have for all app designers.

Keynotopia 

So you have a great app idea but your software chops are a bit lacking? Keynotopia is perfect for you then. By purchasing the Prototyping Bundle for either PowerPoint of Keynote (currently, only $97 dollars!), you get access to a litany of pixel perfect, vector objects that you can use to prototype pretty much any app concept. What's great, is that Keynotopia comes with objects custom made for any device you can think of (yes, there's even Blackberry UI templates included), so all of the spec work is done for you. Use it in Keynote or PowerPoint and let your creativity flow.

Balsamiq

Balsamiq is similar in concept to Keynotopia, as it provides a drag and drop solution to easily build and design mockups for any of your app ideas. What's great about Balsamiq, is the product was designed with quick app-making in mind, so there are pre-made menus that hold all of the objects you will need for your designs. This makes the wireframing process quick and easy. In short, with Balsamiq, you can go from prototyping to rapid prototyping. 

Invision

Invision is used by some of the heavy hitters in the app design world - Twitter, Evernote, Salesforce - to name a few. And they're definitely using it for a reason. If you are working in a bigger team with a lot of cooks in the design kitchen, than this piece of software will prove to be incredibly useful. With real-time design presentation and collaboration tools as well as version control for your designs, Invision makes it easy to design together.

Proto.io

If you are comfortable making static designs but want to make them more interactive, than Proto.io is perfect for your needs. This app makes it incredibly simple for you to animate your designs. But taking it one step further, Proto.io lets you preview your prototypes on actual devices (Android and iOS) without having to install anything. Now there's no excuse for you to not build your dream app.