Will NOT work in multiplayer.
Fire1 shoots a red portal.
Fire2 shoots a blue portal.
You can teleport through either portal to reach new areas.
A test map is included. See if you can beat it without cheating

http://www.doomworld.com/idgames/?id=14686
or
http://gothic.34sp.com/pub/doomwads/portalgun.zip
Enjoy!