By free(site) do you mean the standard warining page => main page => two gallery pages, and nothing more? Or something like a hub?
Freesites submitted to LL's bring in most of my traffic, though the search engines often favor me more than my top LL referrers.
It's hard to say which traffic sources are most productive, but the freesites that get the most SE exposure tend sell more than the rest for me.
I take a slow and steady strategy when it comes to hardlink trades. People often say this is a long-term business, so I treat is as such.
Freesites, and well managed hubs wich point to those freesites, make good long term cash producers.