ZMA is a combination of zinc, magnesium, and vitamin B. A number of manufactures market it- GNC, Vitamin Shoppe, Biotest etc. Here is a quote from the Biotest website about it:
University studies have found that the zinc and magnesium contained in ZMA are absorbed far better than other forms. An independent ZMA study was recently conducted on competitive NCAA football players at Western Washington University. The results show that, in eight weeks, the ZMA group increased free and total testosterone levels by 30%, while the placebo group showed a decrease of 10%. The ZMA group also had a slight increase in IGF-1 levels compared to a 20% decrease in the placebo group. In addition to increasing anabolic-hormone levels, the ZMA group gained 2.5 times more strength and functional power over the placebo group.
Not sure whether it is all hype, but I have noticed some improved performance in the gym while taking it. Just wondering if it makes a difference if I take it during the day instead of at bedtime.