Linked_Portal_Door
Just got and started playing around in the sdk so went straight to my most anticipated feature, linked_portal_door. Made a example map to show what i mean if you don't know.
http://www.youtube.com/watch?v=ahY4E6iu7W0
The level basically a hollowed out cube which each of the walls covered by a linked_portal_door connecting to another random wall.
This has a few bugs still (it was a 30m map including working out how to use linked_portal_door) such as seeing the 'void' through the cracks between the walls but it's really just to show this feature off and hopefully inspire some great level designs, I know I've got a few ideas for it.
Anyway long story short I think just could make some very interesting map designs. I've got a few ideas myself but was wondering if anyone else had any thoughts on them.
Quick addition as I made another map showing walking on the ceiling.
http://www.youtube.com/watch?v=UDH0NoclEWg
download link- Download
Do you have a sample map or tutorial I could look at for using linked_portal_doors? I'm very new to this and haven't been able to get them to work.
keep experimenting with those and you might be able to figure out how my gravity changers work 
Oh, I know exactly how the gravity changes work. What I'm having trouble with is getting a linked_portal_door to work in the first place! 
I've thought of all sorts of interesting things you can do, but first I need to make a simple working one that just connects two regular areas. Then I can dive into the fun stuff.
Here's an idea: Make a long hallway, break it up into segments, rotate each segment so that a different side is the "floor", link the segments together with link portal doors, and now you have a hallway where the direction of gravity keeps changing the further through it you've run.
So... how to I just get a simple linked_portal_door to work?
I would strongly, strongly recommend being careful with these. The idea behind it is pretty cool in theory, but in practice these end up being annoying to the player, at least in my limited experience.
These work against the player's intuition, which violates an important game design tenet that should only be broken if the payoff is extremely worthwhile. If you do decide to do this, I would recommend very clearly indicating to the player what is happening in a simple example and then expanding from there, like both Portal games do at the very beginning-- allowing you to watch yourself going into one portal and coming out the other one, thereby teaching you the basic concept of Portal. I further suggest that any other time you do this you make it very clear to the player that it's happening, again both Portals do this--they use very shiny portal animations on surfaces to indicate what is happening-- something like a specific-looking door, some indicators on both sides, or really anything is should be better than nothing in this case. Hiding transitions from the player is just begging for confusion and frustration.
The confusion caused by these is not helped by the fact that you can see your portals where they actually are in the map, even through walls...
No arguments there. I have a campaign idea I want to try out that requires some serious bending of space as part of the environment. Instead of hiding that from the player, it would become an integral part of the puzzles. If I can make good enough maps, I'll even take a stab at a real storyline w/ recorded dialog and newly-composed music.
And if I can't make good enough maps, it'll be all sorts of fun to try anyways.
I've been searching for any kind of linked_portal_door tutorial and haven't found a thing. Argh. Help!
Sorry about the delay. Wasn't around yesterday.
Download
That's a link to the vmf file for the second map. Now I'm off to try and make a map where you can walk on the walls / ceiling. Maybe 2 walls, the floor and the ceiling for now as i think that'll be a lot easier to make.
Just finished a new map introducing the Aperture Science Gravity Boots!
download
It's really too much fun making these I just wish I had time to make some proper maps myself instead of just tests.
Got another map made up testing controlling them with triggers.
Video
Download
It seems like you need to let them have a 5 second delay before activating them, otherwise they open then close again straight away, unless I'm missing something.
I've got a few good ideas for this such as puzzle maze maps, I just wish i could commit the time to it.