One of my most-used X utilities is not even really a graphical one.
xclip allows putting stuff in the X clipboard for subsequent pasting into any application. I use it to “copy” canned email responses which I keep in a folder as .txt files and then paste them in the browser for sending by email. No need to bother with browser extensions or built-in crappy functionality (gmail canned responses used to just lose my responses every once in a while).