You have seen it time and time again, especially when doing a search on a company name. It occurs when there are three or more relevant results at a particular domain name for the search query. Many SEOs do what they can to get two listings and the more results from link on a particular search query. It helps ensure the searcher clicks on your result. Obviously the more screen real estate you take up, the higher the CTR (click through rate).
Forum discussion at WebmasterWorld. I know this is a simple topic, but I have never covered it before here.