Zemanta is a service worth a look for a site with an RSS feed and fresh daily content. It puts your site in front of blogs that may find that content useful and possibly result in a link.
Your cost is per impression not per link. So this is similar to running a CPM campaign only the target is not keywords and consumers but site publishers specific to a niche/topic.
At one time some popular SEOs were vocal and public proponents of buying text links from brokers, even being indiscrete enough to endorse them from the link broker sites themselves. But when it became slap-in-the-face obvious that buying text links from brokers was not a viable way to "build links" for their clients these so-called experts bought the biggest and whitest hats around and turned to Zemanta to save their butts. That's when viral link projects took off and Zemanta was one of the services powering those campaigns.
I wouldn't say Zemanta is a cornerstone of a link building initiative. But it is a white hat tool for getting your content out in front of those likely to link to it. Your success will depend on how relevant or link worthy the content is.