From Yahoo! Tech:
Tired of paying $1.50 or $2 for every directory assistance phone call? Then don’t. There truly is such a thing as free directory assistance, though it may cost you a little in time and patience. Jot this down:
A Boston-based company called Jingle Networks offers this free alternative to expensive 411 calls via your cell phone or land line. How and why do they do what they do for free? Just as radio and TV shows are supported by advertising, the directory assistance service is sponsored by companies advertising with the service. So, yes, that means you may hear a short, audio ad when requesting a phone number, but not every time.
Worried about privacy? Don’t be:
The company vows on its web site that it does not give out cell phone numbers or other private information.