I would like to install Debian with a desktop other than Gnome. In this case I can't use tasksel but have to install every package with apt-get.
What do I need in order to run Debian with Windowmaker?
To install Window Maker, type
aptitude install wmaker
However it is not a desktop environment. It is only a window manager. It means if you want to have a program for every possible task, you will need to install them separately. Of course you can use KDE, Gnome, XFCE and other desktop application with Window Maker.
apt-get install wmaker