You'll find rubs geared towards the type of product you're smoking. You can use a general purpose rub that'll handle most everything or use a targeted rub. With that in mind, I've got different rubs each for brisket, pork butt, ribs, tri tip, beef short ribs, chicken, turkey, salmon, prime rib, etc. I also have a couple of the targeted rubs for most, a couple general purpose rubs, and a Greek rub.
I get them premade at the store, online and make my own mainly from Paul Kirk's and Dr. BBQ's books as well as others. If I'm smoking ribs, for example, it's kinda neat to look in the pantry and decide whether I have a taste for one of the general purpose rubs or one of the rubs made specifically for ribs. Then there's good old salt, pepper, and garlic. Yea, I've got several steak rubs too.