"Stackers" refers to Stacker 2 or Stacker 3, one brand of "ECA" stack. "ECA" refers to Ephedra, Caffeine, and Aspirin -- a combination that has been found effective in raising metabolism and contributing to weight loss. However, the Ephedra and Caffeine combination also raises blood pressure, and may trigger a heart attack or stroke. As you probably know, the sale of ephedra will soon be illegal in the United States.
I used Stacker 2 for two months last year to break a stall, but I've since decided that the benefits are not worth the risks, since the risks of hypertension increase with one's age (I'm 44). If you're younger or more inclined to risk stroke or heart attack, you can try an ECA stack while you still can buy them. You can find Stacker 2 and other ephedra products at
www.1fast400.com (as well as other places on the Web) -- that's where I get my post-workout protein.
As for name brands, most of the experienced ECA users seem to feel that a combination of generic ephedra, caffeine, and aspirin works about as well as any of the name brands -- the effectiveness lies in the ephedra-caffeine combination, not the "extra fat-burning ingredients" that many of the brands tout.
The new "ephedra-free" stacks typically replace the ephedra with synephrine, which has some similar effects to ephedra, but doesn't (yet) have the reputation of causing a lot of deaths. Most ECA users feel the ephedra-free formulations are much less effective than ephedra-based stacks.
You can find more information about Stacker 2 and other ECA stacks in the Thermogenics thread in the General Exercise forum.