Depends on the driver you are using. For nvidia, check [FILE]nvidia-settings[/FILE] for an option to rotate. For the open source intel/radeon drivers, you can use [FILE]xrandr[/FILE] with the [FILE]--rotate[/FILE] option. Something along the lines of:
[cmd=""]xrandr --output DVI-0 --rotate left[/cmd]
Obviously the argument to [FILE]--output[/FILE] might be different, depending on the driver and if you have multiple monitors.