A sinister set of two portals, the endless fall trap is a relatively simple setup. Two portals, vertically aligned so that falling from the top one causes you to enter the bottom, thus appearing up in the air and falling again form the main focus of the trap.
Being caught in the trap by falling into the bottom portal effectively causes one to fall forever, with acceleration due to the effects of gravity. As speed increases, someone caught in the trap will find it more difficult and dangerous to try and escape the trap.
Third Portal Variants:
Third portal variants have a third portal located away from the first two. The third portal activates a set amount of time after the trap has been cycling through its initial two portal scheme. The third portal then activates, causing the hapless victim to launch out of the third portal at great speed.
1: Third Portal is aimed at spikes.
2: Third Portal is aimed at hard surface. (Floor, wall, ect.)
3: Third Portal causes victim to fall into a pit of snakes and broken glass.
4: Third portal located on the top of a far mountain peak, launching the victim off the summit.
(Note: Inspired by comments on this submission: http://strolen.com/viewing/Recursive_Portal_Trap)