There is a new Hammer editor (Previously knows as worldcraft) at the valve erc Homepage.
changelog

This is the current changelog. It must be stressed that this is a work in progress, not a final release, so your comments and feedback are quite valuable.

added model rendering in the 3D view

added pointfile viewing to the 3D view

modified Half-Life and DoD FGDs to add model viewing support

improved the texture application tool and advanced compile mode dialog UI

separate pitch, yaw, and roll angle compasses added to entity properties

"animate models" toggle in 3d options

adjustable viewdistance for models in 3d options

selectable animation sequences

customizable 3D view background color
notes:

The angle compasses in the entity properties still don't work. They have not been hooked up yet. Note, though, that you can rotate models in the 2D views in the same way that you would rotate a brush model. To easily disable the default 15°, hold down Shift while you rotate an object.

There is an issue that occurs when you load a second map in Hammer. As explained by Matt, the textures aren't being re-generated for opengl (glGenTextures), so the models think they have a texture loaded and call glBindTexture, but get nothing, as the textures arent in there. Just something to be aware of, if you run into the problem.

To make model preview work, you must use the Half-Life FGD (or modify your mod FGD as outlined below) and unpack your PAK file's models folder to Half-Life\\valve\\models (or the applicable mod folder). You can find PAK file utilities in the Half-Life Utilities page. You must also make sure the Mod and Game Directory settings in your Game Configuration settings (accessible throught Tools -> Options) are set correctly.

The new masked-texture model format is supported. The masked areas of these models will be transparent in the editor.