Crafty is a 3D object, material, model and file browsing utility targeted towards Half-Life modders.

Half-Life Model Viewer 2.0 is a brand new tool based on HLMV 1.25 and Jed's Model Viewer 1.36 that provides an entirely rebuilt model viewer.

The Blender Source Tools allow Blender to import and export Studiomdl (.SMD) Data and DMX model files.

A nice, custom, GUI that allows extracting Garry's Mod addon files (.GMA).

GCFScape is a small tool that can open and extract from BSP, WAD, PAK, GCF, VPK, NCP, XZP files.

Very powerful tool that can download items from Steam workshop, unpack/pack VPK and GMA, view models, decompile and compile models.