Disclaimer

While TWHL is a site that hosts both GoldSource and Source modding resources, it mostly focuses on GoldSource. If you cannot find something here, you can visit Valve Developer Community, which has a lot more information about Source modding.

Modding Tools

• Duct Tape - Duct Tape is a project building tool which allows users to make a full mod (while using a few other tools.).
• XBLAH's Modding Tool - XBLAH's Modding Tool is an IDE (Integrated Development Environment, basically a full suite of Tools.), it includes everything you need to make a full mod from start to finish. It comes in 2 editions, the Essentials Edition (free), and the Advanced Edition (Patreon subscription). The tool is split up into different tabs, which represent the different roles in a team. It can make mods for GoldSource, Source and in some cases Source 2.
• VIDE - The Valve Integrated Development Environment (VIDE), is a third party tool, used to embed custom content into a BSP file while featuring a VTF and VMT editor, along with a particle editor, entity lump editor and much more.

Mapping

• Valve Hammer Editor (Hammer 4.x) - Included with every Source game install, along with the corresponding compilation tools.

• Hammer++ - Provides a Hammer style workflow, while adding new features such as a lighting preview, gizmos, prefab support, 3D skybox previews, instances and more.
• Slammin' Source Map Tools - A modified version of the Valve Hammer Editor, also including new features.

Game Data Files (FGDs)

In order to use the point- and brush-Entities, you'll need an FGD File.
Most (if not all) Source games, provide their own FGD file, so you'll have to find them under a specific path.
This path in most cases is: steamapps\common\<game>\bin\<game>.fgd

Compilation Tools

• VCT (Valve Compilation Tools) - These are provided with every install of a Source Game, along with the VHE.
• Slammin' Source Map Tools - These tools come with compilation tools already provided to take advantage of newer hardware and editor features.

GUI Compile Tools

• Valve Batch Compile Tool - This tool is a GUI-based tool which uses VCT to compile maps without using the Hammer interface. This allows the tool to use more of your computer's resources, as you don't need to have Hammer open.
• Compile Pal - Compile Pal is a GUI frontend for the compile tools, which means its also able to run outside of Hammer, allowing for slightly faster compile times like in the tool above. The Features of Compile Pal include multiple compiling presets, batch compiling, and auto-detection of the last SDK that was run.

Displacement (Terrain) Tools

• DispGen - DispGen (or Displacement Generator), is a tool which generates displacements from a heightmap image.
• Terrain Generator - This tool is a bit different. It allows the user to shape the displacements however they want it. The user has full control over the terrain, instead of relying on an algorithm.

Decompilation Tools

• BSPSource - BSPSource is a GUI-based map decompiler for Source games (some people consider it an unofficial continuation of VMEX). Some of its signature features include, being able to decompile multiple maps at once, extracting embedded assets, and various other features. BSPSource is based on VMEX for its decompilation process. BSPSource supports all Source games.
• Valve Map Extractor - Valve Map Extractor (or more commonly referred to as VMEX), is another decompiler, for Source Engine maps, being able to convert BSP files into .vmf Files for any compatible editor to load. VMEX supports all games up to Left 4 Dead.

Additional Map Tools

• Map Analyst - Small Tool for embedding materials, texture, sounds, and other assets into a .bsp file.
• Map Obfuscator - This is an obfuscation tool, used to deter theft of maps. This tool works by renaming entities and making the following of the entity logic difficult. Be aware that this tool may break your map, so always back up your map and make sure you understand the changes and the resulting consequences.
• VPKEdit - This is a tool for viewing, extracting, and packing VPKs. Some people consider this a better version of GCFScape.

Modelling

Model Tools (Non-GUI based)

• NekoMDL - NekoMDL is a fork of StudioMDL, adding some new Features, along with a 64-bit application, faster compilation using a new algorithm, etc..

Model Tools (GUI based)

• Crowbar - Crowbar is by far the most popular of the MDL compilers and decompilers. This tool is used for every source Game. This tool can decompile VPKs, GMAs, and FPX Packages, individually or as an entire Folder. You can even publish the files as an addon onto the Steam Workshop.

Model Converters

• FireSoft Half-Life MDL Converter - FireSoft MDL Converter is used to convert Source models into a format that is understandable by the GoldSource engine.
• MDLDecompiler - MDLDecompiler is a tool, used to decompile MDL files into their individual components (namely an SMD, VTA and QC file). Be aware that the tool is by far not perfect, and that you might have to edit some QC files to be able to recompile the model properly. There are fixed versions of these linked on the same page.
• Propper - Propper is a tool used to make models in Hammer. It works by using a .vmf file and converts it into a model. It also has the ability to create a physics mesh or create a QC file, which compiles the prop to be able to be used in-game.

Model Tools

• 3DS Max
• Blender
• Cinema 4D
• Maya

All of these tools are the de facto standard for modelling, used across the industry. Ranging from hobbyist game developers using Blender to create their models, up to entire companies using one of these programs to make feature-length animated movies. All of these tools have their own plugins to support the Source engine.