Access to computer serial and parallel ports has been important for hardware hackers who wanted to control their creations with a computer. Those ports are disappearing from new computers. The serial port seems to be maintaining a tenuous hold in the form of USB to serial interface devices, many of which are based on specialty chips from http://ftdi.com . This company makes some relatively inexpensive modules like the UM232R which can emulate a serial port in software, that is, your program thinks it is talking to an old standard serial port. One fairly exciting feature of the UM232R is that it has a "bit bang" mode where instead of acting like a serial port it can produce inputs and outputs on some of its pins. For some applications this could replace the functionality of the old parallel port model where individual pins on the port were toggled to produce a given result. Of course, for a few more dollars you can add a microcontroller like a PIC greatly increasing the functionality of the system.
Adding a parallel port PCI adapter to a PC motherboard that does not have its own integrated parallel port does not always give the desired result. Many of the parallel port boards will not map their base addresses to the old standard addresses of 0378, 0278, or 03BC hex. If your application requires these addresses be careful. If anyone has a lead on a card that is available and will do this please drop me a line.
Note also that parallel to USB adapters will not work because they do not present themselves as I/O ports to the software side. There is an exception that I have not tried myself: http://www-user.tu-chemnitz.de/~heha/bastelecke/Rund%20um%20den%20PC/USB2LPT/ul-15.htm.en
Device are not always available and there is no commercial source though the author invites duplication.