For xinerama I am using a window manager with automatic tilling. I have experience only with awesome on slack 12.2. As I know at this moment there is no a slackbuild script for awesome for slack 13.1 (very bad). In other cases all what is by default shipped with Slackware - even twm (I like it). I wish that windowmaker is poorly configured. It really deserve much more attention - if you don't belive me try yourself LiveCD from GnuStep website - cool!