[danilocampos]

And as long as we're geeking out, money existed but not in the Federation. So, not for humanity. Outside cultures had it, which made them an interesting foil for a humanity whose abundance taught them to value other things.

[lotharbot]

There's a little bit of ambiguity. Starfleet didn't use money, at least not explicitly, but there were humans who amassed personal fortunes, and other Federation worlds that had market economies. Money was also referenced several times in various Treks [0], sometimes with a character talking about how humans don't have money, and other times with a character talking about selling something, charging for something, or being willing to spend money for something ("I'd give real money if he'd shut up" - McCoy, referencing Chang's Shakespeare quotes, ST VI)

[0] http://memory-alpha.org/wiki/Money