The following function returns a list of bits (number 1 and 0) from a program, and prints a message if used interactively.

    (defun bits (num)
      (interactive "nNumber: ")
      (unless (integerp num)
	(error "%S must be an integer" num))
      (let (result (n num))
	(while (> n 0)
	  (setq result (cons (mod n 2) result)
		n (/ n 2)))
	(if (interactive-p)
	  (message "%d is %s" num (mapconcat 'number-to-string result ""))

M-x bits RET 5 RET prints “5 is 101”, while (bits 5) evaluates to (1 0 1).

SimpleCalculator provides a different, interactive-only solution for converting numbers: The keys B,H,O switch between binary, octal and hex-mode. M-x calculator RET 5 B prints “Calc=B⇒ 101”

Library Lisp:hexrgb.el converts between hex and decimal RGB values (and also between RGB and HSV color components). – DrewAdams