Currently available only in Firefox and Internet Exploer (the latter still in beta testing), Zemanta’s main goal is to provide to help bloggers find relevant links to their articles by suggesting related content through words and images on the side (after you install it).
One of the great things about Zemanta is that it actually scans the web for Creative Commons content, so bloggers will be able to post useful images without the fear of being sued by an angry photographer/artist (provided they link back that is).
It may also help fend off bloggers block, which can creep up unawares upon authors (especially in the middle of a post).
Unfortunately the only draw back to Zemanta is that the software has the potential to be abused by sploggers, who may find it easier to leech off the backs of a future giant than to purchase the software to scrap content themselves.
That aside, Zemanta may prove its worth, especially if they are able to include video recommendations (hopefully beyond YouTube, although that may be a great place to start).
The software is currently only available in for Blogger, Typepad and WordPress.com and self hosted WordPress blogs (2.0 and up), although they will hopefully consider expanding the software to other blog platforms (such as Moveable Type).
Update: Corrected link for Moveable Type (Thanks Dave for the heads up).