Since I moved to Berlin in September I have been studying German at the Friedrichshain Volkshochschule. The Volkshochschulen are locally funded adult education colleges that offer all manner of classes, including German as a foreign language.
There are endless numbers of language schools in Berlin, and having spoken to friends I have concluded that high fees do not necessarily guarantee high standards. Therefore I recommend the Volkshochschulen to anyone looking to learn, as they are certainly the cheapest.