Matt's got a great example of how the public domain can be a valuable resource, and while he uses an example where it's promoting a band, I think the bigger potential is for corporate use of the public domain.
There's a lot of examples of companies doing this, though it's not something they draw attention to, so it still seems unintuitive. Working at a newspaper, especially online, we're acutely aware of photo rights for publication or posting of images that we get. The first place we go when we find out that we can't use a photo is to the public domain. Images created by most government agencies are immediately ceded to the public domain, providing a rich library of images for all but the most esoteric stories.
Perhaps the best example of a corporate interest leveraging the public domain in this way is MTV's famous Moon Man promotions, which date back to the channel's first days on the air.
After having had the brilliant brain storm of an ever-changing logo, (which has been effectively appropriated by others in the interceding years) the producers in charge of creating promos at the nascent channel were constrained by the limited timeframe and even more limited budget for making a bumper to fit in between the then-nonstop stream of videos.
Their solution, of course, was to appropriate NASA's moon launch and landing footage, presenting the familiar video with the MTV logo composited into the shot on the flag that was planted on the moon's surface. The ads were so effective that MTV's award shows still present Moon Man statues to winners, as a testament to the lasting impression of those early ads.
The subtler point of the success of that apporpriation is that the ads struck a note with an audience by juxtaposing this new, anarchic sensibility against footage that was extremely familiar to the audience. And that audience would never have been as familiar with that footage if it hadn't already been in the public domain. The lack of copyright control on that content increased the value of the derived works for the nascent network.