I already downloaded/tried FreeQDA and WEFT but I am looking for alternatives. The programm should run on Windows and Mac (and if possible Linux as well).
You can download a free trial version of Atlas.ti (http://atlasti.com/free-trial-version/). You can really experience the programme. There is only one limitation: It is not possible to save projects that exceed a certain size, although you can open and use projects of any size. At the end of the day you can make up you mind in an informed way if the software is worth its money.