I once left my computer on 24/7, only shutting it down when I left home for extended periods. Since buying our home and trying to keep it as green as possible I have started shutting it down every night and when not in use. Leaving a computer on contributes 1,500 pounds of CO2 to the atmosphere each year (WARNING: PDF link). If you must leave that quad-core box with 6 SATA drives and SLI video running constantly consider visiting climateprediction.net and joining a distributed computing program that can map climate change and its impact. I am running the application and it seems to be very unintrusive and takes up very little system resources.
Find out more about the project at climateprediction.net.