If you have a LiveCD or an appropriate ISO, IMG or ZIP file then Linux Live USB Creator can use that as a source. But if not then you can have the program download your preferred LiveCD by simply selecting it from a list. And this is nicely implemented, too: it’ll even run a quick speed test on multiple mirrors to find out which one is the fastest for your location.
Of course there’s support for persistence, along with a host of other advanced options. But none of these get in your way: a neat interface means Linux Live USB Creator is very straightforward to use.