Ubuntu’s very own Startup Disk Creator is a good application to create bootable usbs. However, it does have a mind of its own. Sometimes it gets stuck or freezes halfway through.
Ubuntu also ships another application which restores ISO images to disks and can create bootable media. The application is Gnome Disks. The easiest way to get started with the application is to right click on the desired ISO file and select “Open With” -> “Disk Image Writer”.
This will open up the Restore Disk Image window. Select the destination USB drive. Make sure to format the drive before you restore the ISO on to the USB.
The above steps have been tested with Ubuntu 14.04.1 LTS and Fedora 21 ISO images on both Ubuntu and Fedora.