I'm recommending a specific brand in the
Tribulus Terestris thread.
I have good experience with it as part of a stack with other supplements (see my signature).
Note that as demo said, Tribulus does not directly increase testosterone, but it has other effect (also on other hormones like LH apparantely) which help me.
Testosterone in general: Please also see BoneBroth's thread
How to increase Testosterone naturally as going the prescription route is not feasible (doctor needed) or useful (fertility problems by TRT) for everyone.
In general I think a combination works best (pre-pack niacin and certain anti-histamine, post-pack testosterone booster and anti oxidant and anti inflammatory and adaptogens, lifestyle changes, don't masturbate but only have orgasm with a partner, ...), looking for a single chemical subtance might not work.. My life is now quite good and it only kills me if I masturbate :-/ Which can happen after a few days of abstinence.