Freie Universität Berlin's Language Center/Sprachenzentrum (in German) offers pre-semester and summer classes for advanced students. It also has an Independent Language Learning Center (pdf-file), which is a useful resource. To prepare for the TestDaF or for the Deutsche Sprachprüfung für den Hochschulzugang (DSH) German classes can be taken at the Goethe-Institut, at private language schools in Germany, or in the student's home country. The DAAD provides further information on German Language.
Several further education institutions in Berlin offer German language courses. Please check the following website of the Berlin Adult Education Center ("Volkshochschule") (in German), which has branches in every Berlin district and offers a wide range of courses besides languages.
German courses can also be downloaded at: