Spearhead Point Head

There is a spider hanging out on my front porch and I don't know what kind it is...?

The size is just under half a centimeter, the body is dark grey with a light grey/brown pointed stripe that looks kind of like a long spearhead running down the back of it's abdomen . Close to the head, on either side of the stripe there is a small dot of the same color. The body is shaped similarly to a black widow (large butt shaped kind of like an off center drop of water) with long, thin legs. There are also fine lines in the shape of arches that branch off the sides of the stripe that are slightly darker than the rest of the body.
Can anyone let me know what kind of spider this is?
Is there a website that shows a picture and give information about this spider?
Thank you for all your help!
It was suspended from a web it was making between two pillars, about 4 feet across.
and it is hairless as far as I could tell.

http://en.wikipedia.org/wiki/Orb-weaver_spider

sounds like a common orb spider to me

harmless

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