That is Yahoo's search engine spider. Yahoo bought Inktomi a while back and uses their spider (called Slurp) to build indexes for the various SE's that they own, including Yahoo itself, AltaVista, and others. That -- or some related thing -- is probably what you're seeing.
Right now, MSN Search also uses Yahoo's index, but Microsoft is building its own index that will eventually replace Yahoo results. (If you're curious, you can find the new self-spidered MSN index at http://beta.search.msn.com