I know whenever 3D development is being mentioned on TWHL, most often than not Hammer is the preferred subject, and rightly so, since that's what TWHL is at the heart of. Now, I know for a fact that at times, other 3D development applications are mentioned and discussed, nothing wrong with that.
Among these other non-Valve 3D development programs that have been mentioned before, there is one in particular that I have seen being mention 2 or 3 times in some form or other on TWHL: Unity 3D. Now, for those of you here that are acquainted with Unity 3D, know very well that their development suite is not free, it costs a lot of money. Well, that's no longer the case.
The package that was called "Unity Indie" and now is called simply "Unity", will be completely free for developers of virtual worlds and games, which by the way has until now been sold for $199.00, but not any more. Again, it's FREE now! Unity is a set of browser plug-in and toolkit to make games and virtual worlds.
For modelers, Unity can read .FBX, .dae (Collada), .3DS, .dxf and .obj files. You can go to Unity's site and download your copy of Unity and start developing virtual worlds and games with it for free.
Source to this post: http://www.virtualworldsnews.com/2009/10/unity-preps-free-toolset-for-game-dev-community.html
Unity 3D: http://unity3d.com/unity/
Personal Comments: I like the fact that you can export standalone apps or games with it though. It is far more than just a map editor.