Google, Yahoo and MSN. Most webmasters and online marketers spend a lot of their time trying to get listed in the big three search engines. But trying to figure out the algorithms and robot visits for these SE's can mean changing your site for the search engines and not for your customers.
As an example, our partner site HostingGoodies.com has an extremely hard time ranking with Google and Yahoo becuase the keywords are so competitive. Without spending money on PPC campaigns and other advertising, they could never be found on Google.