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What type of bulbs are we talking about? AC incandescent, DC or LED?
You should not connect any light bulbs directly to any port. You might damage the I/O device. You can connect LEDs but must use a limiting resistor.
Actually, using the parallel port would probably be the easiest solution and the simplest program. The USB is strictly a data bus and would require additional hardware. Controling specific RS-232 data lines is harder then with a parallel port.
Unless the light bulb is very tiny, neither the serial port or USB interface is likely to be able to source enough current to illuminate it. You may be able to accomplish that with LEDs.
I would look into the perl module Sys::PortIO, which allows you write & read arbitrary IO addresses. You could use it control the Modem Control Register of a PC UART (and thus the related serial port control signals). You will need to dig into the documentation for 8250-style UARTs to decide which lines are appropriate for your purpose. There is a fairly small and standard list of IO addresses used by serial ports on PC compatibles.
i've done some googling, but not much luck. I'd like to use a real regular light bulb. The bulb doesn't have to get its power from the pc, just controlled.
How much hardware are you prepared to build or buy? There are a few ready-made systems that you can buy that connect to various PC ports. My preference would be a solid-state relay with zero-crossing detection that has TTL level inputs and 120VAC contacts. These could easily be controlled from a parallel port using Sys::PortIO. If you are planning to build your own, the software side will be a small part of the overall project.