Here's a new study put out recently by realtytrac.com. It shows the rental investment returns for specific counties and what you should look for. Their methodology isn't the best, but it had to be done that way if you want to look at the entire country in one glance.
Editor: Crass Cash
This website was created due to the atrociously misguided financial advice that I've heard over the decades. Financial freedom is not intellectually strenuous, but it takes discipline.