I noticed some unsightly black stains on my shingles roof a few months ago. I'm guessing they are the fungus/moss that grows on the north side of buildings and spreads from rooftop to rooftop. What can I use to get rid of them?
The best thing to do is to clean the roof with a 1/3 bleach and 2/3 water solution. You can also use TSP (tri-sodium phosphate) or a commercial product called Jomax  which is a cleaner with a bleach activator. Rinse it with a pressure washer on a low setting. Do this on a sunny day; the sun actually helps the bleach to work more thoroughly.
After you get the moss off you can install a copper or nickel piece of flashing along the peak of your roof. Every time it rains the rain will cause some of the flashing to wash down along your roof and fight the moss before it has a chance to make itself at home.
Remember that this is a potentially dangerous job so if you are unsure of yourself working on a slippery roof, don't take any chances and hire a pro.