Goldsource

Tools

Level Editors

Hammer 3.5
The final version of the original, official Goldsrc editor, including model support.

Sledge
A free, open-source replacement for the Hammer Editor containing new tools and aimed at being compatible with both Goldsource and Source. Created and maintained by our very own Pengiunboy.

J.A.C.K.
General-purpose BSP tool aimed at all Quake-based engines. Runs on Windows and Linux.

QuArK
The Quake Army Knife, another tool aimed at being a universal editor. Don't forget to install the required addons to enable mapping for your chosen HL mod.

FGD Files

In order to use an editor for more than empty rooms, you'll need a fgd!

Half-Life.fgd
Good old-fashioned Half-Life.

Counter-Strike.fgd
Counter-Strike 1.6

Compile Tools

Vluzacn's HL Tools
The most up-to-date and feature rich version of the Half-Life compile tools, with a host of lighting options and optimization entities. Includes 32- and 64-bit builds, so you can get more power if you're running a modern system.

Zoner's HL Tools (Or in 64-bit)
The foundation for Vluzacn's tools, Zoner's tools were open-source and his improvements over the Quake tools the HL SDK shipped with made it the de-facto standard for over a decade.

Viewers

Solokiller's HLMV
Written from the ground-up for modern systems, and with a host of new features. Probably the best model viewer you'll find.

Jed's HLMV
An updated version of the official Half-Life Model Viewer supporting all HL texture features.

Crafty Object Viewer
View models, materials, or even the geometry of compiled maps for both Source and Goldsource. Also supports exporting data to obj, if you need to use map data for creating animations or models.

GCFScape
Browser utility for opening now-deprecated Source GCF files to view or extract content. Also compatible with HL pak files, wads, bsp files with packed information, and Source VPK files, making it an essential tool.

Texture and Sprite Editors

Wally
Create new Texture wads, extract textures from existing ones, or make edits directly in the program.

Sprite Explorer
Viewer and editor specifically for .spr files.

Half-Life Texture Tools
A more recent wad editor that can also create sprites.

Additional Resources

Slackiller's Half-Life Programs
Slackiller hosts a bunch of tools that would otherwise have been lost over the years - including a lot of things linked on this page! Hammer builds, compilers, viewers, and miscellaneous utilities abound.

Valve Developer Union
Tool archival site dedicated to keeping Goldsource modding alive. If you can't find it on Slackiller's site, and it's still floating around, you'll almost certainly find it here.

The Half-Life SDK
Though heavily outdated, this package encompasses everything Valve made available to the general public back in the day, including model and map example source files, early models and map compile tools, and full dll source for Half-Life and multiple official mods.

Half-Life Github
Official github repository for the same code released in the SDK, but with additional updates and bugfixes.

Spirit of Half-Life 1.5 (Or 1.8)
Laurie Cheer's mod that adds a whole mess of quality-of-life features for mod authors. Have buttons and breakable glass move with your elevator, give every NPC a unique model, or fill your map with particle effects. 1.5 and 1.8 are more or less the same feature-wise but each have their own set of minor bugs you'll have to fix on your own using the included source code.

Ambient.Impact.Design
Ambient.Impact's old website has arguably the best HD character models made for Half-Life, it's expansions, and a couple mods, often with source files included for those inclined to make edits.

Source

Coming Soon (tm)