There are already several instructables on making a bench-top power supply using an obsolete computer supply. There's no reason that you have to take the power supply out of the computer to do this; the average computer has plenty of spare power for a few experiments. And the popular +12 and +5V supplies are on nice convenient connectors (designed to attach to disk drives.) Put a panel with a couple of power jacks in a spare bay and you're golden...
With slightly more effort, you can add some fuses and ammeters, both of which are probably good ideas.
With significantly more effort, you should be able to come up with a variable voltage current-limited supply that still fits in a bay.
(Picture from http://www.instructables.com/ex/i/2F7340901BE310299AD7001143E7E506/)