The most common website for flat offers and requests; also available in English, highly recommended:
Made by student government (AStA) of TU Dortmund and FH Dortmund; only in German and with a significantly lower number of flat offers:
There are also some facebook groups like...
Also possible, but not recommended for short-term stay:
http://www.immobilienscout24.de (watch out, they often charge you extra commission!)