Powernowd can lower the clock frequency when the CPU is idle
Use a display adapter without fan. Those with fan usually have higher power consumption than those without.
DPMS will turn off your display when the PC has been idle for a while
Hard Drive Spin Down
Spinning down you harddrives not only will save power but save the life of your drives.
hdparm can be used to spin down internet IDE and SATA drives. First you must disable access times to the drives to keep them from waking up.
Edit your fstab adding noatime and nodiratime.
LABEL=d3 /tv/d3 xfs rw,noatime,nodiratime 0 0
Now, add hdparm statements to rc.local to set the default spindown time, i have used 20mins as a timeout:
vi /etc/rc.local /sbin/hdparm -S240 /dev/sda