Please do your best to identify this spider.  The full price will be
rewarded for eliminating it as a known dangerous spider.  I no longer
have the spider, so I can't examine it more closely.  If I find it
again (there were 3 in my yard) I will save it and post again with a
higher reward with more details when I can look at it with a powerful
magnifying glass.  

Location: San Francisco Bay Area

Thanks!

-C

http://www.agathis.com/spidertoid.htm

This looks to me like an orb weaver.

Some info on orb weaving spiders is available here:

http://www.termite.com/spiders/Garden-Orb-Weaving-Spider.shtml

According to this:
http://www.ces.ncsu.edu/martin/newsletters/newsarticles/twigs/spiders.html
there are only three species of spider in North America that can be
dangerous to man, and a quick image search shows that none of them
look like the pictured spider.

Pink is most certainly right that it is an Orb Weaver.
http://www.rochedalss.qld.edu.au/orbweaver.htm

One just like yours.
http://www.rochedalss.qld.edu.au/spiderphotos04/DSC_2324-Resizeda.jpg

Hi Snorry,

Your spider is definitely an orb weaver.  Below is a link to a a few
shots of an orb weaver in Northern California.  Notice the typical San
Francisco rhododendron in the background.  You have nothing to fear,
except that these enterprising spiders often weave large (~3 feet in
diameter) webs in the path of humans.  These large webs which cause no
harm can cause quite a fright.

http://photos.pequals.com/index.php

I saw the pics before I realised that there were all these comments
further down the page, but my conclusion was - also - that it's an
orb-weaving spider. If genus /Araneus/ is found in the US, that would
be one suggestion.

http://en.wikipedia.org/wiki/Orb-weaver_spider has a bit about them.