There are many major brands out there that focus on notebooks, but the following are the most common. Compaq, Hewlett Packard, and Toshiba have been in the business for quite a while making great laptops. They are going to be the cheaper of the laptops and produce great business/productivity computers. Some of their higher end notebooks will also get into the area of photo/music storage and home entertainment. While they may have some thin and light computers out there, that is not the market that they focus on when producing their laptops.
Another common brand is Dell. Dell is making a statement for some of the best all-around notebook production. You are going to find Dell computers in all the categories, with lots of them focused on productivity and the ability to be thin and light.
Another popular brand is Sony. Sony has been at the top of the consumer electronic field for many years, and it is no surprise that they make great notebooks as well. Sony focuses its laptops on two major fields. The first is the thin and light category. If you see someone working on a tiny notebook in the airport, there is a good chance that it will be black and a Sony. Sony also has some great home entertainment computers as well, with cross-promoting their products with other consumer electronics. The Sony laptops are probably going to be more expensive than the previous four brands.
The final well-known brand out there is the rapidly expanding Apple computer. These are the top of the line music/photo/graphic storage and manipulation notebooks out there, though many people are using them primarily for productivity because of the quick processors they have. Their 12-inch PowerBooks and I-books are also quite thin and light. This brand usually has the most expensive notebooks out there, but they are often well worth the extra money.