A randomized, controlled trial of the effects of remote, intercessory prayer on outcomes in patients admitted to the coronary care unit. (WS Harris, Abstract)
News Article Description of Harris Study
Positive Therapeutic Effects of Intercessory Prayer in a Coronary Care Unit Population (Randolph Byrd) (PDF File)
Spirituality and Prayer - Effect on Cancer
Internet Health Library - Overview of studies relating to Alcohol and Depression
IntelliHealth - Overview of effects of prayer on numerous conditions
I'm not a doctor, but I expect that given the impossibility of knowing if people being studied are being prayed for by others the results of any study will always be suspect. All I'm saying about the above list is that this topic will keep on being debated.
I do know that I've seen powerful examples of the effectiveness of prayer first hand. I've seen concrete answers to prayers for needs both small and great. On the health front, one of our friends was sick with lung cancer, the prognosis was very poor and he needed assited breathing and a feeding tube. We prayed together asking God specifically for a miracle, two months ago I saw him looking as good as ever at a fundraising dinner.
Of course, I've also lost people I've loved despite earnest prayer to the contrary. It's difficult, but even in those times, I've seen God working to bring spiritual healing.
One final note on praying for the sick. The bible is very clear that we should pray for those who are sick:
Is any one of you sick? He should call the elders of the church to pray over
him and anoint him with oil in the name of the Lord. And the prayer offered in
faith will make the sick person well; the Lord will raise him up. If he has
sinned, he will be forgiven. Therefore confess your sins to each other and pray
for each other so that you may be healed. The prayer of a righteous man is
powerful and effective. (James 5:14-16)
My advice stick with prayer, it truly is effective even if the scientists can't figure it out. It's just as well, if they could explain it, there would be some in the medical community that would want to turn it into a clinical excercise to remove God from the equation all together.