One of our posters on the Peak Testosterone Forum listed a thread from a bodybuilding forum where a man took 25 mg of zinc for a couple of months and almost quadrupled his testosterone from 170 to 635 ng/dl.  Now I'll be the first to tell you that you can't rely on a semi-random story on the internet for health and fitness advice. But, as I'll show below, this story is not only not far-fetched but actually backed by the findings of a couple of studies.
In fact, let's look at one study that took four young men and gave them a zinc deficient diet. Their testosterone dropped from a pre-study level of 1176 ng/dl to a zinc deficient level of 311 ng/dl in just 20 weeks.  This is a drop to almost one fourth of starting levels! This seems to match well with the almost four times increase that the above man reportedly experienced from zinc supplementation. However, one might be thinking that this is an artificially created scenario and my not work as well in real life. Well, the researchers read our minds and examined a more practical "real life" scenario:
They took a group of "mildly zinc deficient" seniors and found that their average testosterone levels were very low: 244 ng/dl, which is hypogonadal by any lab standards that I know of. So what happened when they gave them zinc? Their testosterone almost doubled to 470 ng/dl. Notice that this did not magically give them youthful levels. However, the key is that it did boost their testosterone back up to more normal levels for their age group. Not bad for an inexpensive supplement, eh?
The takeaways from this study seem clear and are:
--Zinc deficiency can lead to very low testosterone levels.
--Correcting a zinc deficiency can lead to dramatic increases in testosterone.
--Correcting a zinc deficiency can restore testosterone to "normal" levels
NOTE: You can test for a zinc deficiency using what is called a "taste test." For example, check out the Designs for Health Zinc Challenge
There are other signs that zinc may improve your testosterone levels besides just a straight deficiency. For example, one study took men with long term infertility and testosterone less than 480 ng/dl and put them on zinc. Their testosterone, dihydrotestosterone (DHT) and sperm counts rose significantly.  Of course, it's probably prudent to just get a zinc test done, but this study shows just how widespread zinc deficiencies may be and how other male-related symptoms may be a sign that one is low on zinc.
NOTE: You may also be interested in my page on the The Pros and Cons of the Zinc Taste Test.
Why does zinc boost testosterone? I have never been able to find a solid explanation for it. Of course, zinc is known for inhibiting the aromatase enzyme, which will tend to lower estradiol and raise testosterone. My HRT clinic, for example, will even give a man zinc as "Arimidex Light" just for that purpose. But it's aromatase-limiting powers are quite limited and so this would have a minimal impact on testosterone. Adding to the mystery is that neither the enzyme that converts pregnenolone to progesterone, DHEA to androstenedione or androstenedione to testosterone actually contains zinc, so zinc's testosterone-raising role is clearly secondary. However, since zinc is actually in 100+ human enzymes, its role in human health is critical and it is, therefore, no surprise that it can so dramatically alter one's testosterone.
So will zinc always raise testosterone levels in men? Well, some supplement manufacturers would like us to believe so, but that is probably not the case. I discuss this in more detail on my page on Zinc, ZMA and Testosterone, but the zinc / magnesium combo in ZMA supposedly significantly increased testosterone according to the first study (funded by someone with a financial interest). However, follow up studies have not shown this to be the case and the consensus seems to be that zinc will only boost testosterone significantly in those that are truly zinc depleted.
CAUTION: More is not better when it comes to zinc. Zinc can be neurotoxic and competes with copper. There is evidence that even small amounts of zinc supplements can lead to some rather nasty issues, something I show in my page on The Potential Dangers of Zinc Supplementation.
2) Nutrition, 1996 May;12(5):344-8, "Zinc status and serum testosterone levels of healthy adults"
3) Systems Biology in Reproductive Medicine, 1981, 7(1):69-73, "Effect of Zinc Administration on Plasma Testosterone, Dihydrotestosterone, and Sperm Count"
4) Am J Clin Nutr, Jul 1992, 56(1):1 148-157, "Effects of dietary zinc depletion on seminal volume and zinc loss, serum testosterone concentrations, and sperm morphology in young men"