. use as web server
Or, if you have one of those cute old Macs (like the Classic or SE/30), see macquarium.
if all else fails, they make great objects for tossing out of twentieth floor windows. Defenestrate your old computers - let them go out with a bang.
Also, many old computers are full of perfectly salvageable parts (like power supplies). My favorite is to take the transformer from a monitor and use it to power a CO2 laser.