Horny Goat Weed - quite a name for a supplement, eh? How are you going to explain that to the wife and kids? Well, I'll leave that up to you, but you might want to use the scientific name of epimedium. The important thing to realize that Horny Goat Weed is well worth your consideration, especially for those of you struggling with erectile dysfunction.
Why? Horny Goat Weed (HGW) does everything a guy could want. First of all, it really does make you "horny" at least in rats and has increased their sexual response in anumber of studies.  One study of healthy rats found that a constituent of this supplement, icariin, actually boosted testosterone levels.  Most Horny Goat Weed supplement manufacturers do an extract of icariin along with the plant itself.
That's just the beginning of the good news when it comes to Horny Goat Weed and erectile dysfunction: it also increases your precious Nitric Oxide. In fact, that same compound, icariin, actually blocks PDE5 (and PDE4 which is used by the body for various inflammatory responses).  You may recognize PDE5 is the same phosphodiesterase chemical that Viagra and Cialis act on in order to increase Nitric Oxide. One recent study examined various herbal or herbally derived products including Horny Goat Weed, Icariin and Tribulis Terristris for PDE5 inhibitory properties and found that only the Horny Goat Weed and Icariin were able to inhibit PDE5.  In other words, Horny Goat Weed increases Nitric Oxide by the same mechanisms as Viagra: slowing down PDE's Nitric Oxide consuming capacities This isn't just theoretical: several studies have shown that this supplemetn actually increased both cGMP and Nitric Oxide. 
NOTE: Icariin has many other excellent properties. See this link on the The Power of Icariin for more details.
This same study showed a relaxation of smooth muscle tissues, allowing increased blood flow into the penile chambers.  Obviously, this is a supplement formula for a better erection. A second study showed that administration of Horny Goat Weed was "erectile producing" in rats.  Finally, a very interesting study on embrionic cells in the lab found that it "inducible effects of icariin were partly related to increase in the expression of cardiac developmental-dependent genes, and elevation of the cAMP/cGMP ratio in ES cells, as well as upregulation of endogenous NO generation". Translated that means: Horny Goat Weed actually turns on genes that increase Nitric Oxide output for you steadily all the time 24 X 7.  What a contrast when compared with the erectile drugs that last for between 4 and 36 hours, eh?
You may have noticed that erectile dysfunction related human studies for this supplement are lacking. Well, all I can say is all those rats couldn't have been that wrong. Steven Lamm, M.D. in his excellent book, The Hardness Factor, uses it in his practice and says that many of his patients swear by it. He performed a (non peer-reviewed) HGW study on 38 men. 25 were healthy men and 13 were taking Viagra. About two thirds of the healthy men and about half of the men using Viagra responded positively to the Horny Goat Weed. Of course, this is not a rigorous study, since the placebo effect was not taken into account, but it's a good sign nonetheless. .
Also, it is worth noting that some supplement manufacturers put Maca and Horny Goat Weed together in the same capsule taking advantage of their complementary effects on male physiology. Horny Goat Weed directly boosts Nitric Oxide and libido. Maca, through a different pathway, boosts semen volume and libido.
1) Intl Journal of Impotence Research, July/August 2006, 18:335-342,"Epimedium brevicornum Maxim extract relaxes rabbit corpus cavernosum through multitargets on nitric oxide/cyclic guanosine monophosphate signaling pathwayEpimedium brevicornum Maxim extract relaxes rabbit corpus cavernosum"
2) Urology, Mar 2006, 67(3):631-635, "Effect of Epimedium brevicornum Maxim extract on elicitation of penile erection in the rat"
3) Acta Pharmacol Sin, Mar 2006, 27(3):311-20
4) Asian J Androl, Mar 2003, 5(1):15-8
5) Vascul Pharmacol, Jul 2007, 47(1):18 24
6) J Natural Products, 2008 Sep;71(9):1513-7. Epub 2008 Sep 9, "Potent Inhibition of Human Phosphodiesterase-5 by Icariin Derivatives"
7) Asian Journal of Andrology, 2006, 8:601 605, "The testosterone mimetic properties of icariin"
8) Journal of Ethnopharmacology, Dec 2007, 114(3):412-416, "Effect of lipid-based suspension of Epimedium koreanum Nakai extract on sexual behavior in rats"