Tongkat Ali (Eurycoma Longfolia). One study on rats using Tongkat Ali showed a significant increase in testosterone. [1]
This has not been verified in humans however. One set of researchers in Brazil noted that Tongkat Ali's true powers seem to lie in its ability to halt the "inhibitory effects of estrogen on testosterone production." [2]
They further noted something very important for many guys: "where there is an excess of estrogen or deficiency in testosterone, Eurycoma longifolia jack may be able to act as a remedy for the condition.

Of course, a low testosterone, high estrogen state is what a large percentage of males struggle with in modern, industrialized societies and so Tongkat Ali may be a significant help in these situations.

NOTE: Tongkat Ali is generally sold as an extract and men have been using Tongkat Ali for years seemingly without issue.

REFERENCES:

1) Fundam Clin Pharmacol, 2002 Dec, 16(6):479-83, "Effect of Eurycoma longifolia Jack on orientation activities in middle-aged male rats"

2) Clinics (Sao Paulo). 2010 January; 65(1): 93–98, "The Effect of Eurycoma Longifolia Jack on Spermatogenesis in Estrogen-Treated Rats"

11) Planta Med, 2002 Jul, 68(7):662-4, "The toxicity of some quassinoids from Eurycoma longifolia"

 

1) Maca. We have already covered Maca's sperm and erection enhancing properties in our discussion of Maca and in our Erection Supplementation Guide. But what you may not know
is that Maca has had a reputation for centuries among Peruvians for its libido-increasing powers among male tribal members. And animal studies bear out the ancient wisdom. [1]

In fact, one animal study turned rats into rabbits. Don't believe me? It all started when researchers studied rats and put one male rat in with two virgin female rats. Rat heaven, eh?
Researchers were astonished to find that each male rat entered, i.e. penetrated the female rats over fifty times in a three hour period! [2] When is the last time you did that, mate?
This greatly exceeded the number of penetrations performed by the Maca-less rats. Those rats just couldn't get enough.

REFERENCES:

1) J Ethnobiology, 1981, 1:208-212

2) Urology, Apr 2000, Qun Yi Zheng