MultiSystem

MultiSystem is a powerful tool for locked- and unlocked-bootloader Android devices with many features that at least includes the following:

• Keeps stock system partition safe/rooted
• Permenant root survival with proper use
• MultiROM support via virtual ROMs
• Unlimited number of virtual ROMs
• Booting options to choose stock, primary, or secondary virtual ROM
• Any of the virtual ROMs can work as a recovery replacement
• Flashing multiple ROMs at the same time without a reboot
• Ability to create/install ROMs on Linux to microSD card
• Great performance & battery life on virtual ROMs
• Recovery solution to install ROMs or Mods
• Easy upgrade to newer versions of Android
• Ability to safely apply OTA updates to virtual system
• Permissive SELinux and other kernel tweaks
• Safe flashing that doesn't trip KNOX flag on Samsung devices
• Wrapper script runs via ADB or a Terminal Emulator on device
• APK to manage all MultiSystem functions with a nice UI and extra options
• Management for the best performance & user experience
• Support for all Android devices with microSD card
• Portability to almost all devices
• Compatibility with all Android versions

XDA Thread:

More information can be found here.