I think it depends on the market.  For me, it was ShopCity.com's local shopping platform.  It's a subscription-based model that leads to offering services.  You can see ours live at http://shopbirmingham.com.  After that, we added social media contests and are now adding video ads.  To me, those are the three that have the most promise.  We're going with banner ads as our 4th product, but only because we figured out a way to tie them in to our shopping platform and video product.