Many men know that progesterone lowers DHT (dihydrotestosterone), which could be good or bad depending on your situation. What is not as well known is that progesterone can also lower estradiol levels in us guys. Many men, especially as they age, end up with high estradiol levels - usually due to weight gain - and are wondering how to control it naturally. Of course, the first order of business should be to drop those extra pounds through diet and exercise. But one also might want to pull one's progeseterone levels as this could be a partial explanation as well.
Here is a summary of the research-based evidence that progesterone lowers estradiol by modifying aromatase activity. Aromatase is the enzyme in fat tissue that converts some of your testosterone into estradiol. This is why overweight men tend to have higher estradiol and lower testosterone: the more fat tissue, the more aromatase.
I will mention that all of the studies below are in women. Furthermore, it is not really that common for men to take progesterone and then monitor their estradiol, and so it is difficult to even find confirmation out on the forums. However, we had man on the The Peak Testosterone Forum who, when asked if progesterone lowers estradiol, wrote, "Yes it does. I proved it on myself." 
Here are some of the studies that state the same thing:
1. Fat Cells. An in vitro study shows that progesterone inhibits aromatase in a dose dependent manner, i.e. the more progesterone, the greater the inhibition:
"In fibroblasts derived from human adipose tissue, aromatase induction is observed after exposure to 1 M cortisol in the presence of serum or platelet-derived growth factor (PDGF). Progesterone suppresses this induction in a dose-dependent manner, 10 M resulting in complete inhibition. A reduced cortisol concentration (0 1 M) concomitantly reduces the progesterone concentration required for effective inhibition (10 100 nM). This effect of progesterone is specific, as neither the release of cellular enzymes nor aromatase induction by dibutyrylcAMP, which acts independently from cortisol, are affected." 
NOTE: Progesterone seems like the most controversial of the big gun, common hormones for men. It has some stellar results as you can read about in my page on Progesterone and Erectile Dysfunction and The Benefits of Progesterone in Men. However, it also has some decided risks, which I discuss in my page The Potential Dangers of Progesterone.
2. Aromatase Activity. One study took pre and post menopausal women, in order to get a wide range of hormonal values, and basically evaluated all their hormonal data points beside aromatase activity. What they found was that aromatase activity was not correlated with testosterone or cortisol, but was inversely related with progesterone. This means that the more the progesterone, the less the aromatase activity. 
3. Cancer Breast Cell Line. One study looked at a breast cancer cell line that produces high levels of aromatase and found that a metabolite of progesterone (20alphaDHP) is an aromatase inhibitor. It did not find progesterone itself to be one however.  And don't get hung up on the fact that this is a breast cancer line - they use it for rapid screening of AIs (aromatase inhibitors). 
Do males have significant amounts of 20alphaDHP? I had trouble finding confirmation of that fact but did find this:
"These results show that, contrary to the progesterone concentrations, no significant difference exists in the concentrations of 20 alpha-DHP and 20 beta-DHP between the blood from the umbilical vein and arteries. Furthermore, no significant difference could be found in the concentrations of P and 20 alpha-DHP between the sexes in either the blood from the umbilical vein or from the umbilical arteries." 
Regardless, it seems clear that a solid amount of evidence indicates that progesterone inhibits aromatase. (Of course, it would be nice to have a study or two in men confirming this fact.)
Also, I am not encouraging anyone to necessarily try progesterone. It does have some risks, which I list here: The Potential Dangers of Progesterone. And there is a potential backdoor route: supplemental pregnenalone can raise progesterone in some men.
1) Anticancer Res, 2008 Jul-Aug, 28(4B):2129-33, "The anti-aromatase effect of progesterone and of its natural metabolites 20alpha- and 5alpha-dihydroprogesterone in the MCF-7aro breast cancer cell line
2) Biochem Pharmacol, 2008 Jul 15, 76(2):208-15, "MCF-7aro/ERE, a novel cell line for rapid screening of aromatase inhibitors, ERalpha ligands and ERRalpha"
3) Acta Endocrinol (Copenh), 1975 Nov, 80(3):569-76, "Progesterone, 20 alpha-dihydroprogesterone and 20 beta-dihydroprogesterone in mother and child at birth"
4) Journal of Endocrinology, 1998, 158:401 407, "Progesterone inhibits glucocorticoid-dependent aromatase induction in human adipose fibroblasts"
5) J Steroid Biochem, 1986 May, 24(5):1033-9, "Aromatase activity and concentrations of cortisol, progesterone and testosterone in breast and abdominal adipose tissue"