Apparently what this plugin does it that it finds related pictures, tags, links and articles to your post content and proposes them for you to use. I am already using the Photo Dropper WordPress plugin which can find for me Creative Commons licensed content in flickr that is related to my post, but Zemanta promises to take this a step further.
Currently it comes in the form of either a browser or a server plugin. The first form would be appropriate for someone who does not want to load up his blogs with extra scripts and generally blogs from the same place always (say, home). The second form, for which I went for, is better for someone who might blog from 2-3 different locations.
As I’m using it, it looks quite good. I especially like the automatic tag suggestions, and while there are other plugins that can do the same, it’s good that you don’t need 4 different installations. The only thing that wasn’t obvious is that you need to save your article at least once before suggestions start coming in. However once they start, they update every 300 words to give you stuff more relevant to what you are typing in.
So I think I’ll keep it. I’ve already installed it in various of the blogs I manage and I think it will be especially useful for people who just don’t have much time to look around for images or think of tags.