MilkShape 3D
From Valve Developer Community
MilkShape 3D is a small modeling and animation software package written by Mete Ciragan. It was heavily used by aspiring modelers/animators when creating models for HL1. MilkShape 3D is probably best known for it's support of a large variety of file formats,and become a useful converter from one file format to another. Milkshape 3D supports Half-Life and Half-Life 2 .SMD files.
Note:Version 1.8.0 and up, no longer destroys vertex weighting.