All my searches results:
- NetRadiant is based on Gtk 1.5.0 and adds some good things; Still in dev; Tested, it moves entities, maybe because of their origin values not right?
- ZeroRadiant is based on Gtk 1.4.0 and adds some good things; Also called Gtk 1.6.0; Dev stopped?
I have a working NetRadiant but can’t get ZeroRadiant toi work.
Links:
NetRadiant Home
Ah, Google seems to have find something for ZeroRad:
http://forums.icyjumps.com/viewtopic.php?f=7&t=650
Will test 
Edit:
It needs to add a pack for ET, looks really different. ZeroRadiant/installs/ folder have the games folders. As it is a download link from UT there’s the UT pack, needs a ET pack, and maybe RtCW pack as well… Will look into it, if anybody have not done it yet?
Edit 2:
Current builds, I tested here and no one worked:
http://zerowing.idsoftware.com/files/radiant/developer/1.6.1/
Edit 3:
Start it, select “Quake III Arena and mods”, then I had ET… I will try to fix the fact it can’t find my shaderlist.txt
Edit 4:
Starts no more \o/ I give up with this one. I has an interesting interface.
Edit 5:
What is this?
http://www.splashdamage.com/forums/showthread.php?t=21253