I recently recevied a very interesting email from an ADHD reader who had done a lot of research on supplements to help with his condition. What he found was that some of these significantly helped with libido and/or erectile strength. The first of these was the granddaddy of all supplements: Vitamin C. He also mentioned phosphatidlyserine and fish oil as well.
As you can see below, his general comments were that all three increased libido but phosphatidlyserine also improved his erectile strength:
"I haven't tried all three together so far, but definitely will later on. If I have fish oil alone, libido goes quite noticeably - so far as erections go, I'll have to let you know when I resume taking it (I'm testing other substances these days and don't want any interference from other supplements). On fish oil, I feel a little calmer, but don't really notice much from a mood perspective, but libido does go up for sure. On PS, libido, mood, well-being and erectile function all go up noticeably. This is the single best OTC substance I've found to work for me so far - depends on your natural state as I mentioned in my previous email. Other guys feel horrible on fish oil or PS."
CAUTION: Always talk to a physician first, especially if you are on medications or have any kind of medical condition.
Why would these help so much with libido and/or erectile strength? I believe - and thanks to the reader who pointed out the beneficial effect on the HPA Axis/Cortisol of these three - that there are two primary reasons, which I have listed below.
1. Acetylcholine. It turns out all three of these can boost or restore acetylcholine (at least in some circumstances) and acetylcholine actually helps dilate arteries.  Of course, nitric oxide is the big gun, but acetylcholine plays a role as well. The research has concentrated on the nitric oxide connection - can you say Viagra and Cialis? - but acetylcholine can help relax those penile arteries and get blood flowing.
2. Cortisol Reduction. We have already discussed Vitamin C's cortisol-reducing powers in my link on Why Take Vitamin C? However, phosphatidylserine and fish oil also have studies showing similar results. 
So is there any evidence that these three will improve help with erectile dysfunction? Well, there certainly is and I outline these in my page on Vitamin C and Erectile Dysfunction. As it turns out, Vitamin C is a pretty good Blood Flow Increaser and is worth considering in your arsenal.
Phosphatidylserine, by the way, is a famed nootropic, i.e. "brain booster". It is present in many foods. The reader took 100 mg/day, in order to keep it line with what you might get through diet. It is a somewhat expensive supplement but you can get it through Amazon: Natural Factors Phosphatidylserine 100mg 60-Count.
The reader also uses Ester-C (500 mg), same as myself. The fish oil that I use is Ascenta Nutrasea Omega-3s, which comes to the health food supermarket where I buy it in refrigerated trucks. I always bite open the first capsule in the container and have never had a fishy or rancid taste. In fact, two of my children actually take it by biting open the capsule since they are not too good on swallowing pills whole yet.
CAUTION: You should also read about Some Potential Risks of Vitamin C as well. To play it safe, it should probably be consumed only when you eating low fat.
1) J. Nutr, Nov 1 2001, 131(11):2951-2956, "Oral Administration of Soybean Lecithin Transphosphatidylated Phosphatidylserine Improves Memory Impairment in Aged Rats"
2) Prog Neuropsychopharmacol Biol Psychiatry, 1989;13 Suppl:S77-88, "Nootropic drugs and brain cholinergic mechanisms"
3) Climacteric, 2003 Sep, 6(3):238-47, "Vitamin C improves endothelial function in healthy estrogen-deficient postmenopausal women"
4) J. Nutr, Sep 1 2002, 132(9):2506-2513, "Dietary Fish Oil Increases Acetylcholine- and Eicosanoid-Induced Contractility of Isolated Rat Ileum"
5) Diabetes Metab, 2003 Jun, 29(3):289-95, "Fish oil prevents the adrenal activation elicited by mental stress in healthy men"
6) J Int Soc Sports Nutr, 2008; 5:11, "The effects of phosphatidylserine on endocrine response to moderate intensity exercise "