I think it's some kind of orb-weaver spider (Orbiculariae, baby!), but I'm not certain. If anyone knows for sure, please come and school me.
It's been out back for about a month now. I discovered it by walking into its huge web during the day. Ever since then I've been always careful to check things out when I'm out there, make sure the unthinkable doesn't happen: walking into its web and the sucker hitching a ride. A few weeks back that's exactly what happened. Mike G. was there. I walked outside at night and screamed. Mike thought there was a little girl in trouble out back. I'd walked right into the web (she rebuilds it periodically, always in a different spot) and my little-huge friend ended up crawling across my face. Luckily no one died. And we got the spider to safety, back up near its tattered web.