Since C is the only language in which I can claim some experience, my programs will naturally be in it. However the logic is simple and can easily be ported to any other language.

/*********************************************************/
/******Simple program to write to and read from port******/
/*********************************************************/
#include <stdio.h>
#include <dos.h>

void main()
{
  int val; 
  printf("Values currently in port");
  val=inportb(0x378);       //read the port
  printf("%x \n",val);
 
  outportb(0x378,0x0F);    //write some value to port

  printf("New values in port");
  val=inportb(0x378);      //read port again. should be 0F 
  printf("%x \n",val);
}

This program utilizes the dos.h header file. The functions inportb() and outportb() found in this file are used for direct I/O operations in DOS. The above program first reads the existing values latched in the port using
val=inportb(0x378);
This may be any junk value. The function "inportb" is of the format return-value=inportb(port address).
Next we output a value to port using
outportb(0x378,0x0F);
We write the value 0F(0000 1111) to the port as the function is of the form outportb(port address, out-value). Now using a multimeter or an LED circuit we can see that the pins 2 - 5 (D0 - D3) are HIGH, nearly 5V and the pins 6 - 9 (D4 -D7) are LOW, nearly 0V.

Now that you have seen the basic I/O functions required to handle data transfer from or to the parallel port, surely you can create programs to suit your needs.

That much was for the DOS user. For the Linux user I/O operations are generally more difficult owing to protected mode operation of the OS. However simple data transfers of the above type can be handled relatively easily. The header file to use instead of <dos.h> is <asm/io.h>. The functions to use are outb(value, port-address) and inb(port-address). For more details refer Linux I/O port programming mini-HOWTO by Riku Saikkonen.

By the way if anything goes wrong, please don't blame me. Be careful with grounding. It might be a good idea to shutdown your system before connecting your hardware to the port. You might damage it otherwise. Also try to use a buffer IC in the external circuit to interface with the port.

And thus we come to the end of our first tutorial. In due course I might come up with others.