Just finished up using one of my favorite and what seems to be little well known web service, the Term Extraction from the folks a Yahoo! You feed it some content and it returns a list of significant words from the content. This simple service can be incredibly useful.
For instance I recently used this service to scan an article on a site and using the words that it returns, as tags to pull relevant photos from Flickr to go along with the article. So in this case I used it to pull tags from a body of text, very cool. You can also use YQL to make calls to it as well.