I can't count the number of times I've needed to quickly rot13 or atbash something, or de-rot13/de-atbash something someone sent me. Well, I can't count because I can't remember. But I know it was at least fifteen times in my life. More than enough to write a shell script to handle it, regardless!
So I give you rot. rot is nothing more than a front-end to tr(1). It allows you to feed it either command-line text or stdin, and returns the modified text to stdout. Currently three substitution modes are available: simple rot13, alphanumeric rot13 (default) and alphanumeric atbash. Additional modes are easily added by anyone who knows how to put additional cases in a bash case statement, and the default is user-adjustable by a configuration variable in the head of the file.