This is one of the best articles ever written about grip strength and building your grip.
However...
I must disagree with Panzer about his assessment of the New York Barbells grip machine. I feel that this machine, although it can only hold 150 pounds, is the best grip machine for a true beginner looking to build a powerful grip.
I recommend the machine to most grip trainers for it's simple construction, it's ease of use, and it is very affordable. IMHO, only when someone is honestly ready to go to "the next level" in terms of grip training should they invest in a more expensive grip machine.
Here is the link to the NYB machine:
http://www.newyorkbarbells.com/9165.html