DHEA, vitamin D, magnesium, and zinc have been seen to raise low testosterone levels. Few other supplements seem to help at all.
A few trials support the use of coleus-forskohlii, but its numerous potential adverse effects make it a risky proposition, especially for older people.
You might also have heard of d-aspartic-acid, but its promising initial trials were followed by several others that found that it didn’t increase testosterone.
Some other supplements, such as ginger and Eurycoma longifolia, might support testosterone levels only in infertile men (or in men with testicular damage).
Many other herbs, such as horny-goat-weed, have not even been studied in humans yet.
Finally, keep in mind that a supplement can benefit libido (as do maca and, according to a small number of studies, tribulus-terrestris), mood, or energy levels, yet not affect testosterone.