I have posted rentals. I am in NYC area. I was never requested to pay a fee.
Yes of course since, unless you're a broker, your rentals posts clearly don't fall into the (already mentioned) paid categories:
Posting fees for jobs in 18 metro areas, brokered NYC apartments, and therapeutic services.
Being from NYC, you shouldn't have any problem in understanding your language
In case you're still not convinced, here is an article which explains how (and how much) Craigslist makes (big) money out of its site:
Craigslist Rakes It In
And it only deals with apartment brokers.