What are some of the best simple tools for building mobile applications fast?

http://www.alphasoftware.com/products.asp

http://www.app-press.com/#overview-sub

http://www.apparchitect.com/

using xamarin studio is the simple way to develop any platform of mobile devices

i think the best way to use native tools .

for example if you need to build Application for Android use java langue with android studio and so on  

Cordova , angualr js and bootstrap css

Dear,

 I've been using ionic for more than1 year and I have built more than apps till now, some of them are on google play store now, it's easy and uses angularJS, also it comes with a stack of ready css and js components, in addition of learning guides. And now the ionic creator has been launched, which you can use to create apps prototypes.

The best of all is that ionic framework is an open source for developing hybrid mobile apps with web technologies.

Quoting from ionic's website:

Ionic is the beautiful, open source front-end SDK for developing hybrid mobile apps with web technologies.

Features:

• Performance obsessed
• Angular & Ionic
• Native focused
• Beautifully designed
• A powerful CLI
• Fun to learn
• Built by nerds (like you)

More than code. Ionic is an ecosystem.

You'll find a suite of mobile development tools and resources at your disposal that make Ionic the complete mobile dev package. It's the best way to build apps. Period.

Ionic is now on beta version2.

check the links below:

Ionic Framework

Documentation

Ionic Creator

Learn Ionic

the best is java language with android stdio 

I can suggest you to use Appzillon to build mobile applications. It wont much time to build applications.

Java for android , Swift for IOS

http://www3.biznessapps.com/demo-app-design/

http://www.techworld.com/picture-gallery/apps/18-best-mobile-application-development-platforms-tools-3375307/#9

java, xml,ajax, html/css, bootstrap