This application can help you understand the basics of how you should use the UNIX chmod BASH command under the hood. With this utility you can set the bits permissions individually and see immediately the effects of how the permissions are changed in the command line and what is the corresponding octal number to use. More importantly, how these bits are converted to octal numbers.
In this app you can also see off-line a great tutorial about chmod by Dave Eisenberg’s, tutorial licensed under the "Creative Commons Attribution-NonCommercial-ShareAlike 2.5 License".
In the section "About" of the app, you can share your opinion either directly through the app using the twitter or by using the e-mail.