accessplace and zettai come top in my immediate area, followed by the google maps result - in which I'm at the top, the only one with my details filled in on it! If you haven't done that already, get on the case now, it's free!
I think a lot of people look at the web address in green writing on google, and scan down until they get past the free directories and find a "proper" looking web address to click on.
Only ever had one enquiry from a free online site, cant remember but I think it was ufindus or touchlocal. Most of them are piggy-backed off the thomson directory, so if your company details change in the Thomson be prepared to have to contact about 10 free sites to get them to update your details, they don't seem to keep updated.