By now you have probably seen the multiwindow feature in devices like the Galaxy S4 and the Galaxy Note 2. The feature allows you to use multiple screens at once on your display. This is great for productivity and also for the Awesome factor! Native Multiwindow does have its limitations. Only some apps are included. These are normally stock apps and tw apps. There is an app that will allow you to modify your multiwindow and add any app you like. You can even enable every app in your app drawer for use with multiwindow. Some roms do this natively like BeansTown106's Jelly Bean's Build, and Sbreen94's Hyperdrive rom, but if you aren't in to running Roms on your device this app will do the trick.
This app works with several Samsung Devices including the Galaxy S3, Galaxy S4, Galaxy Note, Galaxy Note 2, Galaxy Note 8.0, and the Galaxy Note 10.1. The app does require that you are running at least Android version 4.1.2, and you will need to be rooted. It should also be noted that if you are running a custom rom this app will only work on TouchWiz based roms. It will NOT work on any AOSP Roms like Cyanogen Mod or AOKP. The free version comes with the goods, but if you grab the Pro version you get even more features. Pro features include opening multiple instances of some apps, Fast reboot which cuts reboot time virtually in half, detect new app installs and prompt user to enable for multi window, and backup and restore your enabled apps list.
Play Store - Free
Play Store - Pro