I've seen it wrong too many times. Time for some prescriptive smackdown. People write codexes, codici, codi, and all manner of linguistic blasphemies. The correct plural of codex is codices. That is all.
Source: Merriam-Webster's dictionary.
*Post Edit* Please don't recite the crazy Latin rules for plurals. I am fully aware of them and if we were living in Imperial Rome you would be totally correct on the different declensions. However, in this case codex is a loanword, and as a result when we're speaking English the correct plural is codices. GW tends to say codexes which is technically wrong but I'll survive that misuse.
While we're on the subject, the correct plural of octopus is not octopi. It is octopuses. Octopus comes from Greek, and the -i ending is Latin. If you want to get really technical the correct plural is octopodes but nobody uses it.