I figured out how to create an infinite fall, since that's something I've been trying to do since I started the game. It's pretty neat.

1) Switch to first-person view. (1 key.)
2) Create a blue portal in the ceiling somewhere, anywhere. (Preferably not over lava.)
3) DO NOT MOVE AT ALL from this point on.
4) Look down as far as you can look, then create a red portal (right where you're standing, right underneath you.)
5) You'll fall through it, and land immediately underneath your blue portal.
6) Repeat Step 4.
7) Enjoy falling forever. Man, if only this were possible in real life...

NOTE: I left my computer for 20 minutes and came back and I was still falling. It's safe to say that the fall lasts forever.

You get more of an illusion of speed if you do this in low-ceiling rooms, and more of an illusion of height if you do it in huge atria.