Here is a place to post some prefabs for level design that people may find useful. I'll start:
Name: Pipes
Description: Two pipes, about as wide as the Quake guy, one of which has a turn in it. It took me quite some time to get this to look right, so hopefully someone can get some more use out of it.
http://entar.quakedev.com/misc-files/pipes-prefab.map