As daven said FOOD, FOOD and more FOOD. A couple months ago I thought there was a magic supplement that would put hueg mass really fast. At the time I was eating around 2000 cals a day, definatly not the way to put on mass, even with a "magic supplement". Thanks to the Prof I was pointed in the right direction, eating 4500+ cals a day and only taking a multi. The best thing to put on mass is food, lots of food.
If you have your diet on track and are shovelling heaps food down then protien powder and creatine could work to your advantage, but are not neccessary.