Adding explicit thumbnails (featured images) to your content takes time. Even if you don’t add your own photography, you’d still need to go to search.creativecommons.org or so. This plugin hooks into the thumbnail URL and changes this with an image from the Flickr API, based on the tags in your post. If Flickr couldn’t find anything, it will return an empty string. I have achieved good results, with posts that all of a sudden had a rather adequate thumbnail.
[Flickr images might have licensing issues, so I recommend to use this plugin only on personal sites]