Neriene radiata (Filmy Dome Spider).
 
 
 
Size: approx. 20-30 mm. Photo: 08/09/11, 2 pm.


This is probably a Filmy Dome Spider (Neriene radiata). This species produces a dome of fine silk. The spiders then position themselves upside-down under the web. The second link has photos that match this specimen perhaps slightly better than most posted in the bug guide. The body without legs is approx. 5 mm long. 
 

 
bug guide (this photo):
http://bugguide.net/node/view/560226
eurospiders:
http://www.eurospiders.com/Neriene_radiata.htm
spiderling:
http://www.spiderling.de/arages/Fotogalerie/Galerie_Neriene.htm