I’ve heard it translated both ways, but which do you use?
Paz en la Tierra a los hombres de buena voluntad.
Except if you’re not reading it in Spanish, of course 
(The Spanish version can not be translated into English as “goodwill to men”, only as “to men of good will”.)
I always thought it was “… and good will to all men.”
It was originally in Greek (Luke 2:14): “ἐν ἀνθρώποις εὐδοκίας” or “ἐν ἀνθρώποις εὐδοκία”.
Then in Latin: “hominibus bonae voluntatis” or “in hominibus bonæ voluntatis”.
To work out what it is, you need to deal with those (including the variations).
…and goo will to all men.
Being the benevolent sort I am, I am now going to fix Annie’s typo in the thread title, from “eath” to “earth.”
I ain’t 'fraid of no Doug.
I’m ok with it, Ellen, because you reminded me of the groovin’ dance hit of 1984.
Those are two very different meanings and don’t seem interchangeable. Non-Christian that I am, my source is the Charlie Brown Christmas Special.
The KJV seems to be “good will toward men” while the rest seem to be variants of “to men of good will.”
When we were kids, my brother thoguht it was “gruel to men.” He wanted to know what the women get to eat.
Linus and the KJV have it as
“and on earth peace, good will toward men” (Luke 2:14)
Well duh. Peas! 
The new English version of the Roman missal, due in 2011, will change this phrase back to its correct translation “peace to people of good will”.
“Hominibus” is either ablative or dative plural. “Bonae” is either dative or genitive singular, or nominative plural. “Voluntatis” is genitive singular.
So I guess we have “bonae” modifiying “voluntatis” as a genitive phrase modifying the dative “hominibus”. “To men of good will” is pretty much a word for word translation.
Change “voluntatis” to “voluntates” and you get “bonae voluntates” as nominative plural phrase, and thus might get “Good will to men” as a translation.
Research suggests something similar actually happened in the Greek: