Function Machine
http://nlvm.usu.edu/en/nav/frames_asid_191_g_3_t_2.html?from=category_g_3_t_2.html

This technology plays to the common metaphor that functions are like “machines”- you put a number in (input), you get a number out (output). This applet allows students to put a given set of numbers into the function “machine” and have the machine generate an output. Students must also try and decipher the function “rule” in order to determine missing outputs for inputs.