This spectacular web has been appearing daily in our yard. It goes right across our front door, so we have been knocking it down because it freaks the kids out a bit. I have a policy of not killing spiders that can't kill me. The spider is a garden orb spider, about 2 inches in size, and harmless.
The web building technique is amazing as I have read here. In our spider's case, the top 'bridging thread' spans from our gutter, across our little access road, up a very high retaining wall andto the top of a lamp post on the nearby main road. It is unbelievably high and so strong!