Laptops with the Longest Battery Life?

Yi Ding asks: "Recently, I have been investigating laptops for clients, and the majority of the complaints about current laptops is battery life. Most laptops just don't have enough juice to even finish a single DVD or write an article for 4-5 hours in an internet cafe. Of course, one can lug around extra battery packs, but it's a pain and often defeats the purpose of having a laptop in the first place, portability. What have your experiences with battery life been and where can I find the longest lasting, reasonably robust, laptop?"