[piedamaro]

I made a script: it fades unfocused windows and gives 100% opacity to the focused one. Pretty neat but somewhat resource hungry. Inspired by eutotrans.rb by Daniel Forchheimer.
It depends on bash and standard X tools only, and should work with other WMs too (tested with metacity).

#!/bin/sh
# This is a script that uses transset-df or any other transset-
# patch that can take an window id as argument to allow
# opacity to follow the focused window.
# The focused window get opacity 1.0 while all others
# have opacity set by the var 'FADING' in this script

# It is licensed under the GNU GPL license
# http://www.gnu.org/licenses/gpl.html
# This program is written by: Luca De Rugeriis
# inspired by the work of Daniel Forchheimer (eutotrans.rb)
# Last update: 2005-12-29

FADING=0.85
TRANSSET="/usr/bin/transset-df"

#fade all windows
WINDOWS_LIST=`xwininfo -root  -children | grep -v panel | sed '1,6d' | grep -v 0x1000021 | cut -d " " -f 6`
for i in $WINDOWS_LIST;do
        $TRANSSET --id $i $FADING
done

LAST_FOCUSED=0

while (true); do
        FOCUSED=`xdpyinfo | grep focus | cut -c 16-24`
        if [ $LAST_FOCUSED != $FOCUSED ]; then
                $TRANSSET --id $FOCUSED 1
                $TRANSSET --id $LAST_FOCUSED -t $FADING
                LAST_FOCUSED=$FOCUSED
        fi
        sleep .1
done

To launch and kill it I use modified version of frodon's script:

#!/bin/bash

if [ "$(pidof xcompmgr)" ]
then
        killall xcompmgr
	for i in `ps ax|grep eutotrans|grep -v grep |cut -d " " -f 1`;do
		kill $i
	done
	sleep .5
	killall metacity

else
        xcompmgr -fF -I.02 -O.02 -D6 -cC  -t-4 -l-8 -r5&
	~/bin/eutotrans-clean.sh&
	sleep .5
	killall metacity
fi

Have fun!