Probably the next most frictionless method of distributing a Shiny application to R users is by hosting either a .zip or .tar file of your application, either over the web or FTP. You will need somewhere to host the file, and then users can run the application using runUrl(), as follows:

runUrl("http://www.myserver/shinyapps/myshinyapp.zip")  

Of course, you can distribute a .zip file any way you like—your users need to only unzip and then use runApp() from within the directory, just as you do when testing the application. You can email the file and distribute it on a USB drive for any method that you choose. The disadvantages of this method are that your users have to unzip the file themselves (although this is unlikely to confuse many R users), and any changes made to the application will also need to be distributed manually.