Air travel: Where to book a flight

Information on airfares
My experience (Jochen) showed that cheapest airfares to San Francisco cost between 480 to 560 Euro. I found never a cheaper flight than 480 Euro.
Usually airfares are cheapest in germany when booked in advanced, but this is not usally the case. Sometimes flights closer to the departure day are cheap as well. But if you book a flight less then 10 days in advance, the prize will unlikely to be cheap.

German agencies:
http://www.travelocity.de/de_DE/tcy/home.do
http://www.fly.de/
http://www.travel-overland.de/
http://www.mcflight.de/
http://www.travelchannel.de/
http://www.expedia.de/
http://www.opodo.de/

US agencies
http://www.orbitz.com/
http://www.expedia.com

Sometimes it is cheaper a to book a flight from germany to the us using www.orbitz.com
This of course depends on the exchange rate and on taxes.
You can do this using orbitz, but it is not possible via expedia.

airlines in germany
http://www.klm.com/travel/de_de/index_default.html
http://www.lufthansa.com/online/portal/lh/de
http://www.airfrance.de/
http://www.britishairways.com/travel/home/public/de_de

airlines in the us
http://www.nwa.com/
http://www.southwest.com/
http://www.united.com/
http://www.continental.com/web/en-US/default.aspx
http://www.delta.com/home/index.jsp
http://www.aa.com/

My experience is, that booking via the airlines directly in the us is mostly cheaper than using an agency.