I have some Sixt discount codes, but I don't know how to check rates with them online!? Sixt likes you to have a personal contract number, but I have yet to see decent rates below the Platinum level. I have two gold cards from Sixt = two contract numbers, but rates are mediocre.

Booking through the Happydigits link gives 20 % off even on online rates:
http://www.e-sixt.de/cgi-perl/rental/or
You can get the Happydigits card for free if you have an address in Germany (+ maybe its neighbour countries)

Microsites are another way to book with Sixt online:
Lufthansa: www.e-sixt.de/lufthansa
German Wings: http://www7.germanwings.com/de/partner_mietwagen.html
DBA: http://www.dba-vorteilswelt.de/index.php?site=36
Helvetic: http://www.e-sixt.de/partner/helvetic/
L'tur: http://www.e-sixt.de/apps/travel/hc/...partnerNo=9120
Air Berlin: http://www.airberlin.com/site/cobran...151435&lang=de