Many supplements marketed at males are lumped together as “testosterone boosters.” For example, the herb Eurycoma Longifolia Jack (a.k.a. Tongkat Ali) is often labeled as one, although others have pointed out that it doesn’t boost testosterone per se, and have dismissed it purely on that basis.
Well, let’s look at what Examine.com’s page on Tongkat Ali has to say about this:
Eurycoma (Tongkat Ali, LongJack) is a pro-fertility agent and aphrodisiac that appears to have a large body of evidence supporting this role and some evidence suggesting it may be an anti-estrogen and pro-erectile agent. Lack of evidence for testosterone boosting
A possible mechanism of action of Eurycoma Longifolia on increasing sperm count and for increasing spermatogenic cell count is via suppression of the actions of estrogen 
High estrogen in males is associated with decreased testosterone. Presumably, a decrease in estrogen would bring the testosterone to estrogen ratio in balance to yield a healthy, masculine vitality in men. Not to mention virility.
So just because a supplemental herb like Tongkat Ali doesn’t directly boost testosterone levels does not mean it doesn’t have its applications in hormone-balancing. The key to understanding this is just paying careful attention to the language used in the documenting literature.
I should mention also that anything that reduces stress could be technically a testosterone-booster. Stress and its accompanying hormone cortisol, negatively affects testosterone in males (and estrogen, for females):
The researchers suggest these findings reveal new insights into the physiological effects of stress and how they may play a role in fertility problems. According to research, chronically elevated cortisol levels can produce impotence and loss of libido by inhibiting testosterone production in men. In women, chronically high levels of cortisol can produce severe fertility problems and result in an abnormal menstrual cycle. 
So by this logic, I would consider a stress-reducer like L-theanine to be an indirect testosterone booster:
In subjects given a mental arithmatic task after 200mg of theanine or placebo, the increase in anxiety, stress, and heart rate that occurred in placebo was attenuated with theanine.
Basically, everything that lowers stress, promotes relaxation and/or reduces symptoms of depression also have to do with improving testosterone levels. All of the science is there, you just have to squint a bit and cock your head to the side to see it.