Having a Nexus is a good option for various third-party development and testing. Latest entry in the development is MultiROM. MultiROM lets you install and boot multiple operating systems on an Android device. You can install any available Android ROM like Firefox OS or Ubuntu on your Nexus 5 with MultiROM.
via XDA Developer’s.
According to Developer
MultiROM is one-of-a-kind multi-boot mod for Nexus 5. It can boot any Android ROM as well as other systems like Ubuntu Touch, once they are ported to that device. Besides booting from device’s internal memory, MultiROM can boot from USB drive connected to the device via OTG cable. The main part of MultiROM is a boot manager, which appears every time your device starts and lets you choose ROM to boot. You can see how it looks on the left image below and in gallery. ROMs are installed and managed via modified TWRP recovery. You can use standard ZIP files to install secondary Android ROMs and MultiROM even has its own installer system, which can be used to ship other Linux-based systems.
* Multiboot any number of Android ROMs
* Restore nandroid backup as secondary ROM
* Boot from USB drive attached via OTG cable