The market for gaming laptops is growing as laptops become more powerful and it is possible to make upgrades to them constantly. Here is a list of the companies that make dedicated gaming laptops:

Voodoo, Alienware, Mayhem, Vigor, Sager, ASUS, Rockdirect, M-Tech, Dell

Most of these “brands” of gaming laptops are actually made by the same 2/3 Taiwan companies and just marketed differently. 
Alienware is well-known for the stunning and ‘out-there’ designs of its laptops, but dont assume they are all design and no substance. They have some of the most powerfu gaming notebooks around.
Rockdirect (a British company) also make very high quality laptops, however they are expensive. The main reason given for the high prices relates to the quality of the materials that they use.