Search for cheap flights:
www.skyscanner.net
http://www.expedia.de/
http://www.opodo.de/
http://www.traveltopia.de/
http://www.travelocity.com/
Airlines:
German:
http://www.lufthansa.de/ (Miles & More)
https://www.condor.com/de
US (register here first: https://esta.cbp.dhs.gov/):
HP now US http://www.americawest.com/
WN: http://www.southwest.com/ listing: +1 (866) 792 75 29, (+1 (866) 359 79 67)
US: http://www.usairways.com/ listing: +49 180-300 06 09, +1 (800) 325 99 99
UA: http://www.united.com/ listing: +49 69-50 07 03 87, +1 (800) 825 54 78, (+1 (800) 864 83 31)
http://www.delta.com
http://www.continental.com
European:
www.buzz.de
www.ryanair.com
www.go-fly.com
http://www.gexx.de
http://www.airberlin.de
http://www.germanwings.com/
http://www.easyjet.de
http://www.dba.de
British Airways, ID-travel
http://flyzed.com/ für AB, AY, BT, DE, EY, FC, HG, JK, JP, JZ, KF, LH, LO, LT, LX, OS, OU, OV, SK, TG, WF