Is there a such thing as a free reverse 411 service?  The answer is yes, kind of.  You can very easily do a reverse phone number lookup using a Google or MSN search, but if they have never entered there phone number in an unsecured page on the net, you'll never find their number. 
There are many reverse 411 services out there that charge a small fee.  Normally, you input the mystery phone number into a query box and in order to get the owner's info, you are charged a small fee.  This is worth it if the number in question is sparking a burning curiosity within you.
Often times, reverse cell phone lookup services will even offer you a yearly  subscription for a small fee.  This is a great option, because now you get unlimited reverse 411 searches for a full year.  This is a great option for small businesses or people who have a lot of phone traffic or prank callers.