Free classified ads can be useful and they are free, but don't get any illusions about making a big score. Most of the free classified ad websites rarely get much traffic and your site is likely to be lost in a sea of other ads. Even paying for services with 1000's of listings won't be very productive. Better to spend your ad dollars on targeted pay per click ads. The other misperception is the value of the incoming links. These are not read by the search engines because they are usually on non spidered webpages. Paid classified ads will undoubtly perform much better for you - they can usually prove their traffic as well.