I have been changing switches and plugs and in one of the receptical boxes I found the neutral wire pigtailed to the side of the box, and when I installed the new receptical there was no circuit until I did the same, pigtailed to the box... is this safe?

Credendovidis

No, it is not.
The neutral should NEVER be connected to any housing or used as earth, simply because it is not earth.
The neutral can have any voltage around 0 volt, depending on the load on the used transformer in your supply line.
Disconnect a neutral line from housing, and use a connection block for interwiring.
Sometimes a wrong wire colour is used for earth or neutral wire. Earth should always be green or green/yellow. Neutral should be blue (3 phase Eur) or white (2 phase US).
.
If necessary draw in another wire. If you do I advice you to draw besides the new wire also an extra wire (the last one to draw in whatever you need next time). Always handy!
I am a qualified electrical contractor (in Europe - not in the US).

Stratmando

Check the boxes(switch, recepticle or light)closest to the bad one and see if disconnected their, could even be bad or burned at panel also.
Credendovidis, In the US, the System is actually Called "(Single Phase).
As it is One phase, with neutral in the center. Take Care, Keep up the good work.

artosie

Excelent... I will check the other boxes near it.. it is an old building and running another wire is close to out of the question. I might have to cap the lines and put a steel plate over the box.

Thanks for the quick and informative information! you are a great help