Androstenedione - banned by the FDA in April 2004. 6-OXO - banned by the FDA in July 2006. One over the counter testosterone booster after another banned by the FDA. A fella could quickly get the idea that there are no herbs that can increase testosterone according to the research. After all, doesn't the FDA just instantly shut them down?
Actually, the opposite is actually the case: there are many studies using herbs that show an increase in testosterone under certain circumstances and I have listed them below. Now it is true that the FDA tends to shut anything that boost testosterone in the 50-100% range.
However, below I outline some of the herbs that likely work (to a lesser extent) under certain circumstances.
CAUTION: If you think you have low testosterone, get checked out by a (good) doc. Low testosterone is a serious medical condition that has long term consequences, including risks for diabetes, heart disease and erectile dyfunction. Also, if you are on any medications or have an existing medical condition, talk to your physician before making any change
1. Fenugreek. One study on 30 young males found that 500 mg of Fenugreek increased total and free testosterone by 7% and 12%, respectively.  Now this isn't a big increase, but it also seems to proportionately inhibit the normal conversion to estrogen and DHT. Furthermore, Fenugreek is dirt cheap - see for example Fenugreek Seed 610 mg 90 Caps - and so it may be worth a try. Fenugreek has many other excellent properties, including blood sugar/insulin control and libido and growth hormone augmentation.
Is Fenugreek safe? Well, it should be as it is eaten as a common food and spice in many eastern cultures. Now there is a fenugreek extract, Testofen, out there that is very effective in improving libido and has its own following. However, the extracts may boost prolactin, not something a guy necessarily wants to do. Here is an example of the extract on Amazon: Testofen Fenugreek Extract (300 mg 60 Caps).You can read more about here in my link on The Benefits of Fenugreek.
2) Korean Ginseng. Does Ginseng increase testosterone? Well, it may in certain circumstances, especially guys that are struggling in the androgen/fertility areas. Several studies have noted that Ginseng boosts cAMP levels. One follow-up study observed that boosting cAMP in turn increases testosterone, LH and DHT levels in men with low sperm counts.  (And study on rats has also observed an increase in testosterone levels.) 
NOTE: One very small study of college age men showed that testosterone (and growth hormone) levels were not significantly raised by Ginseng.  My guess is that Ginseng produces a small boost in testosteorne for some men. It has the added benefit, for men with erectile dysfunction, of generally increasing nitric oxide and hardness factor. For more information, see this link on How Ginseng Can Boost Erectile Dysfunction.
Some Ginseng manufacturers have been caught with lead in the their products. To get around this, one should probably play it safe and go with a brand that has been tested by an impartial testing company such as Consumer Lab. This brand, for example, was found to be lead free and to deliver the active ginsenocides specified on the label: Solgar - Korean Ginseng Root Extract, 60 veggie caps .
Did you know you can inexpensively do your own testing for many hormones? The industry leaders is Discounted Labs..
DOSAGE: In one of the studies where the men experienced significant improvement in erectile dysfunction , the men took 900 mg of Korean Ginseng three times daily. Smaller amounts can be taken effectively by many men of course and several studies have used 600 mg 3 times daily.
3) Horny Goat Weed. Horny Goat Weed has a PDE5 inhibitor, a la Viagra and Cialis, called Icariin. Most Horny Goat Weed supplements on the market have an extract of Icariin to boost its power and efficacy. This may be good for testosterone in some circumstances as one study showed that Icariin increased testosterone levels in animals with damaged reproductive systems. 
Icariin has the added benefit of generally improving erectile function in men with erectile issues. For more information, see this link on The Benefits of Icariin and Horny Goat Weed. Also, for those interested: Source Naturals, a supplement manufacturer that seems to have a good reputation, has a Horny Goat Weed extract with 10% Icariin: Source Naturals Horny Goat Weed Extract (Epimedium) 1000mg, 60 Tablets .
4) Tribulis Terristris. This herb is somewhat controversial because, although initial evidence showed that it did increase testosterone, one one recent and study showed the opposite.  What gives? Well, one important thing to note is that the study showing that Tribulis did not significantly increase testosterone was on younger, healthy men. These men already, generally, have abundant testosterone, so do we really expect Tribulis to help in this category?
Tribulis may very well affect men with low testosterone or with certain types of hypogonadism, which may explain the many anecdotal reports of it helping significantly. Furthermore, Tribulis has some excellent properties, such as raising libido, that make it a very likely bedroom booster even in cases where it only marginally helps testosterone. For example, other studies show that it increases both libido and nitric oxide. Some of these other qualities I have covered in some detail elsewhere in my links on Super Sexual Herbs, Aphrodisiacs (Male Libido Supplements) and the How to Increase Testosterone Naturally Guide.
Tribulis is typically packaged in very expensive formulations in health food and bodybuilding stores. However, the herb itself is not expensive: Ultimate Bulgarian Tribulus (750 mg, 90-Count) or NOW's brand NOW Tribulus (1000mg, 45% Extract, 180 Tablets)
DOSAGE: The dosage in the above study was 10-20 mg/kg daily. For a 180 lb man, this works out to be between about 800-1600 mg daily. Many men take smaller amounts, more in the 400-800 mg per day range.
5) Tongkat Ali (Eurycoma Longfolia). One study on rats using Tongkat Ali showed a significant increase in testosterone.  This has not been verified in humans however. One set of researchers in Brazil noted that Tonkat Ali's true powers seem to lie in its ability to halt the "inhibitory effects of estrogen on testosterone production."  They further noted something very important for many guys: "where there is an excess of estrogen or deficiency in testosterone, EL 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 probably the most reliable form is LJ100 as it is, at least, backed by certain agreed-upon concentration levels. I am a little cautious about this product, though, because some of its components have shown toxicity.  However, the counterargument is that men have been using Tongkat Ali for years seemingly without issue.
6) Zinc (ZMA). There is one supplement, ZMA, a form of zinc that has one study that shows a boost in testosterone. Although, a follow-up study showed no significant benefit, it seems like that zinc deficient men may get some benefit in testosterone levels, if nothing else from improved sleep. For more information, see my link on Zinc and ZMA. ZMA also has an absorbable form of magnesium, which is worth its weight in gold.
CAUTIONS: Another supplement, high potency resveratrol, has also been shown to raise testosterone.  However, it has also been reported as an estrogen agonist, i.e. estrogenic.  Forskolin is another supplement that has a study showing a modest increase in testosterone.  Unfortunately, little is known about its medium, or even short term effects.
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"
3) Panminerva Med, 1996 Dec, 38(4):249-54, "Effects of Panax Ginseng C.A. Meyer saponins on male fertility"
4) 8) Systems Biology in Reproductive Medicine, 1982, 8(4):261-263, "Effect of Panax Ginseng on Testosterone Level and Prostate in Male Rats"
5) J Strength Cond Res, 2002 May;16(2):179-83, "Effects of ginseng ingestion on growth hormone, testosterone, cortisol, and insulin-like growth factor 1 responses to acute resistance exercise"
6) Asian J Androl, 2006 Sep, 8(5):601-5, Epub 2006 Jun 5, "The testosterone mimetic properties of icariin"
7) Arch Pharm Res, 2008 Jan;31(1):83-7, "trans-Resveratrol relaxes the corpus cavernosum ex vivo and enhances testosterone levels and sperm quality in vivo"
8) Proc Natl Acad Sci U S A, 1997 Dec 9, 94(25):14138 14143, "Resveratrol, a polyphenolic compound found in grapes and wine, is an agonist for the estrogenreceptor"
9) Obesity Research, 2005, 13:1335 1343; "Body Composition and Hormonal Adaptations Associated with Forskolin Consumption in Overweight and Obese Men"
11) Planta Med, 2002 Jul, 68(7):662-4, "The toxicity of some quassinoids from Eurycoma longifolia"
12) J Ethnopharmacol, 2005 Oct 3, 101(1-3):319-23, "The aphrodisiac herb Tribulus terrestris does not influence the androgen production in young men"
13) The Journal of Urology, Nov 2002, 168(5):2070 2073, "A Double-Blind Crossover Study Evaluating the Efficacy of Korean Red Ginseng in Patients With Erectile Dysfunction: A Preliminary Report"
14) British Journal of Clinical Pharmacology, Oct 2008, 66(4):444-450, "Red ginseng for treating erectile dysfunction: a systematic review"