Qwalk is a tool for converting between different 3D game model formats, most importantly MDL, MD2, and MD3 (Quake, Quake II, Quake III). Some modelling software has import/export functionality for these formats, but in most cases it is incomplete, especially if you want to export to MDL or MD2. Qwalk's goal is to implement all features of each of these formats.
The current version of the Qwalk is a command-line model converter. Qwalk also comes with a simple OpenGL model viewer. Later on, these will probably merged into a GUI-driven viewer+converter.
MDLs can be imported and exported with full functionality. Each and every dusty corner of the format specs are supported. It's not recommended to use this tool to tweak MDLs (that is, import from MDL then export to MDL), because there may some loss of precision or "drift" of vertex positions, and because this program doesn't export the "front/back" skin mapping, which means that if you import a model which uses traditional front/back skin mapping (that is, almost every MDL in existence), some vertices will be unnecessarily duplicated.
MD2s can be imported and exported with full functionality. When an MD2 is imported, the program will look for the external skin files referenced by the model. So, make sure you are in the right directory (e.g. "baseq2") when you run the program, because most skins have a path like "models/monsters/soldier/skin.pcx". Alternatively, you can manually specify a texture using the "-tex" command-line parameter.
MD3s can be imported with full functionality. Exporting is currently not implemented at all. MD3s currently do not automatically import any textures at all. You'll have to use the "-tex" command-line parameter to manually specify an image to load as a texture. The PCX, TGA, and JPEG formats are supported. Also note that MD3s which use more than one texture file aren't currently supported. Tags are rendered in the model viewer as tri-colour axes, but they aren't useful for anything yet.