Sex - it seems so simple. Well, until you hit middle age, right? Then some guys find they need Cialis or Viagra or Levitra to boost their Nitric Oxide for increased blood flow. Still other guys need prescription testosterone, i.e. Hormone Replacement Therapy, to rebuild those muscles at the base of the penis in order to trap and move the blood. Finally, there are still other guys who have all the above chemical mechanics going on but have no sexual desire. Their libido just isn't there any more. After all, when you're the one saying, "Not tonight, dear", you just might be in trouble, eh?
And, of course, you can have any combination or percentage of the above, but, either way, it spells trouble for life in the bedroom. That's why it is ideal if you could find a herb that hits what I call The Big Three Bedroom Essentials:
Interestingly enough, Viagra, Cialis and Levitra really only (directly) do one of these: raise Nitric Oxide. In other words, these are strictly One Hit Wonders. NOTE: Hormone Replacement Therapy will only hit two out of three of these. It will definitely help with #3 by strengthening the pelvic floor muscles and testosterone definitely influences libido as well.
So what if I told you that there are herbs that work on all three of the above simultaneously! Well, Tribulis Terristris actually does. It is probably not going to dramatically raise any of these three, but it is well-known for increasing testosterone (by increasing leutenizing hormone) and an animal study showed it increasing both nitric oxide and libido.  NOTE: You can read more about Tribulis Terristris in my links on Aphrodisiacs (Male Libido Supplements) and the How to Increase Testosterone Naturally Guide.
Another herb that has studies increasing all three of these is Korean Ginseng. Ginseng is best-known for it's nitric oxide boosting powers, which I cover in my Erectile Supplement Guide, but it also has been shown to boost libido and testosterone (in men with low sperm counts).  Again, Ginseng is not going to send any of these through the roof, but it is rather remarkable that one herb will likely help with all three. (NOTE: You can read more about it in this link on Ginseng and Erectile Dysfunction: )
Really, Korean Ginseng and Tribulis Terristris are the two herbs that will likely improve all testosterone, libido and nitric oxide. However, there are other herbs that will improve two or more of these. One is the Horny Goat Weed, that definitely raises libido and nitric oxide and may even increase testosterone somewhat as well. You can even add Horny Goat Weed and Maca together creating even stronger libido. If you add Kegel Exercises with it, you will further boost pelvic floor and penile muscle strength, the real goal of boosting your testosterone when it comes to erections.
Finally, Tongkat Ali (Eurycomia Longfolia or LJ100) probably does #2 and #3. There is one study showing an increase in muscle mass , which likely comes from somewhat of an increase in testosterone and a number of animal studies showing increased libido and sexual activity in animal studies. 
1) Brit J Sports Med, Oct 2003, 37: 464 70. "The Ergogenic Effects of Eurycoma Longifolia Jack: A Pilot Study"
2) J of Ethnopharmacology, Nov 12 2009, 126(2):308-313, "Influence of Eurycoma longifolia on the copulatory activity of sexually sluggish and impotent male rats"
3) Ann Acad Med Singapore, 2000 Jan, 29(1):22-6, "Proerectile pharmacological effects of Tribulus terrestris extract on the rabbit corpus cavernosum"
4) Panminerva-Med, 1996 Dec, 38(4): 249-54, "Effects of Panax Ginseng C.A. Meyer saponins on male fertility."