Hammer - Source engine mapping tool. Part of the Left 4 Dead 2 Authoring Tools in Steam > Library > Tools
Crowbar - Most up-to-date Source engine model decompiler. Decompiles mdl and related files into smd and related files. Also has a model compiler interface.
MDL Decompiler - (outdated) Source engine model decompiler. Decompiles mdl and related files into smd and related files.
Señor Casaroja's Engineering Oasis - Noesis tool
Ninja Ripper - Utility to extract / rip 3D models (mesh), textures, and shaders directly from the running game.
3D Ripper DX - Capture all geometry, textures, and shaders rendered during single frame.
SourceEyeCalc "source eye/eyelid conversions for a .qc file instead of using qceyes or by hand on a calculator"
Blender - 3D modelling tool
Autodesk 3ds Max - 3D modelling tool
Blender Source Tools - Blender plugin for SMD and DMX import/export
Wall Worm Model Tools - 3ds Max script for SMD import/export
ZeqMacaw's Gmad Extractor Batch File - Unpacks entire GMA file into a folder with same name as the GMA file.
Doktor haus's batch file for compressing VPKs to 7Z - Compress each VPK file in the same folder as the BAT file into a 7z file.
Batch Files for L4D2 Unpack and Pack VPK - [Made by Doktor haus and ZeqMacaw 10-Mar-2016] Unpack all L4D2 VPK files in same folder as the BAT file and all subfolders. Pack into VPK files all subfolders in same folder as BAT file.
Notepad++ - Text-editing tool; supports plugins; can edit binary files