For rot13, I've found it particularly helpful to have an alias set up to translate the contents of the clipboard. I basically always have a terminal on screen, so to read rot13 text I just have to copy and run a single command. On OS X:

    alias rot13='tr a-zA-Z n-za-mN-ZA-M'
    alias pbr13='pbpaste | rot13; echo'

(echo adds a newline to play well with bash; on linux I use xsel instead of pbpaste.)