This seems handy. Description from the app page:
DriveDroid allows you to boot your PC from ISO/IMG files stored on your phone. This is ideal for trying Linux distributions or always having a rescue-system on the go… without the need to burn different CDs or USB pendrives.
DriveDroid also includes a convenient download menu where you can download USB-images of a number of operating systems from your phone.
You can also create USB-images which allows you to have a blank USB-drive where you can store files in. Another possibility is to use tools on your PC to make a bootable USB-drive out of the blank image that DriveDroid created.