I don't know that I'd classify us as a traditional freemium offering. The only difference between the free and paid accounts is privacy. In any case, this has worked out extremely well for us. In exchange for free access to the site for open source we get a tremendous amount of exposure. In essence we are trading free accounts for free advertising. I think this approach has been critical to the rate of our growth, both for free and paid accounts.

Virality couples well with certain types of freemium models, as long as you're getting significant exposure from the free accounts to make the cost worthwhile.