I have thousands of black looking things with lots of legs. About an inch long. They are on the foundation of my house, in the driveway, and on my porches. They are lot more at night. I see them on top of each other now like they are mating.I live in Georgia. I sprayed sevin dust about a month ago, but now they are back ten fold.[What are they and are they harmful?

They sound like centipedes or millipedes basically harmless.. though some of them in jungles can get VERY big and be poisonous.. I doubt yours are poisonous. We have the same creatures here and I just ignore them unless they are in the house.. after all they are only bugs :D
Try this link
Centipedes, Millipedes & Pill Bugs (http://www.backyardnature.net/1000legs.htm)

Sounds like a millipede swarm. If so, you are due for a couple of months of invasion. Instead of Sevin try using a spray containing Bifenthrin in a six to ten foot band around your house. That should at least keep them out.

Is this your perpetrator?

Millipede and Centipede Management Guidelines--UC IPM (http://www.ipm.ucdavis.edu/PMG/PESTNOTES/pn7472.html)
Click on pix to enlarge.

Back home we called 'em wireworms, but that's just a catch'all.
Dad use ta' give me a nickel for every ten I crushed, then Mom would give me heck because of the smell.
Just lookin' at 'em brings back memories of the smell... :(

If you can find tobacco dust lay a line around what you don't want them to cross, works for 'bout a week, unless it rains, then you need to re-apply.

Don't know if liquid nicotine or sand would work.

K