Good Question.
If you download the r_speeds tutorial and have a look at the .rmf file in Hammer you will see a trigger_teleport brush and a info_teleport_destination entity. The combination of these two create that effect. The portal is just a sprite added to liven things up. That is one way to get the effect, the other way is to use the trigger_camera entity to make it look like you have been somewhere.
I think SlayerA is writing a tutorial on Teleporters, I might do one on simple cameras...one day.
As for the custom game short cut, depends what you are trying to acheive, but if you are trying to create your own custom game you might want to have a look at 69thVlatitude: http://www.vlatitude.com/tutorials.php?tutID=43
because they have a few really good tutorials on Custom Game stuff. The other place to look for step by step instructions is FireBinders site... Lot's of really good stuff there: http://home.att.net/~firebinder/halflife/proj_04.htm
PS: I did answer your question about Half-Life being the best... it's in the !Stuff forum