DriveDroid is an amazing app that allows you to boot your PC from ISO/IMG files stored on your phone. No need to burn CD's or use USB flash disk!
DriveDroid also includes a convenient download menu where you can download USB-images of a number of operating systems from your phone (like Mint, Ubuntu, Fedora, OpenSUSE and Arch Linux). Around 35 different systems are available in this app.
You can obviously create a blank image, if you want. This app uses the features of the kernel that might not work properly on all devices, though, so be careful with using this app.
App comes in two versions, if you want to support the dev, definitely buy the full one!