Yahoo Slurp Spider.

Right quick explanation:
When you search for things on Yahoo (or any other search engine, but we'll go with Yahoo for the explanation), Yahoo doesn't trawl the entire then and there to find what you want. What Yahoo does is crawl in advance and grab a copy of every page on the internet. It can then index those pages, so that when you search Yahoo checks their index to see whether a page exists that matches your search terms.

So what you are seeing is the Yahoo script sending out requests to ask for every page we have one by one. All search engines do this.

However, other search engines might use a single machine to do it so many requests only
show up as one guest user. Yahoo uses any machine in their system that has a spare moment. So many Yahoo machines send requests here and appear as many guest users.

Google actually send more requests, but as they only show up once no-one realises. Yahoo ask for about 4,000 pages each day.