As someone mentioned above, orb weaver. Many varieties, mostly harmless. Generally build their webs at night and hide, if not completely take down their webs, during the day. (They hide from one of their biggest enemies during the day, the wasp. Many species of wasp love to wrestle them down and paralyze them with a sting before stuffing them in a hole underground or a birthing chamber within a nest. They then lay an egg on the still living but immobile spider who will be eaten alive when they egg hatches.)
I've seen some of these orb weavers get large enough to build webs that stretch across entire roads; small ones, generally dirt, but still. When you're drving down a road late at night and feel the need to actually increase your speed so your car doesnt get stuck in it, it's too big. lol
Oh, and it's not a "garden spider" as someone mentioned. This is:
Notice the striking pattern and the zigzag formation in the center of the web.
http://texasento.net/Argiope.jpg