"It is a good code."
Translation:Es un buen código.
If you put "bueno" before the noun, it drops the "o". So it would be "bueno" if you put it after: "un código bueno".
This http://spanish.about.com/cs/grammar/a/whereadjective.htm says that when you put an "adjective of appreciation" like buen/buena first, it emphasizes the adjective, like adding "really" in English. Hmm, I guess that means the English version could (should?) be "It is a really good code."