Originally Posted by Nayyi

It is true that it could be fatal if one would directly connect a circuit, with an external power source, to the parallel port of the PC.

Depending on your load, it can be fatal to you or another people that touches the PC

If the external power source and the PC have a common ground connection, the power supply should be isolated from the mains (ie it must have a transformer)

If you must control something that is not isolated from the mains you should use optoisolators between the PC and your load.

Use safety precautions, remember that line voltages (110V or 220V) are dangerous

Originally Posted by Nayyi

There will be no inputs that will go back to the PC's parallel port yet there aren't aven diodes to protect the PC from feedback. The parallel port will be driving a DC Motor, a Stepper Motor, an Electromagnet and a flashing LED

As dknguyen posted, only one of your loads (the led) is "safe" for your PC.

Both motors and the electromagnet should have, at least, diodes in parallel to protect the port from the EMF spikes they generate.

Without the diodes they "can be fatal" to the PC.

Quick word of warning! - it's VERY easy to kill the parallel port in your PC, and as they are now part of the motherboard, it's not cheap to repair it (adding an extra card may not be possible, and may not work for everything either).