Do guitar amps give out 12 volts power like a computer power supply?

As a rule, no. There are some amps that will run off of 12 volts, but they don't output power. In a lot of electronics that plug in the wall, the AC voltage (in the US 120 Volts), the 120 volts is converted down to a lower voltage through a transformer. In many cases it is converted down to 12 Volts (DC), because that is a convenient voltage for consumer electronics (along with a lot of other reasons). In theory, you could tap into the 12 Volts in your amp and uming the transformer is powerful enough, use it for powering something else. You wouldn't want to do this though. If you don't understand the electronics, opening up the back and playing around can be dangerous. While 12 Volt DC is fairly safe, If it plugs into the wall, there are voltages that can kill.