Hammer is a 3d level building program, and is free if you have Half-Life 2.
You might be able to find a standalone copy of hammer or world craft lying around still, for free if you look. But it probably won't be the most recent version.
Just use Torque Constructor if you don't have Half-Life 2. You should probably use it anyway, since it was made for the engine, and doesn't require you to import custom textures through some strange and needlessly complicated method.