For non-Valve games and mods, consult the mod website. Often the FGD is included in the mod install folder.
Vluzacn's ZHLT v34 - 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. Official home on the Sven Co-op forums.
Nem's Batch Compiler - An interface for the compile tools, if you don't like using the command line or compiling through your editor.
HLFix 0.81b - A tool that converts a Hammer RMF to MAP files. In the process, it performs a number of geometric operations to fix common problems with brush geometry. Some custom builds of Hammer include these fixes by default, and the modern Hammer alternatives do not require HLFix.
Please do not use decompile tools to steal other people's maps! However, they are useful tools for seeing how certain entities are used.
To actually create texture images, use your favourite image editor such as Photoshop, GIMP, Paint.NET, etc.
kHED (or older free version) - A basic - but easy to use - modelling tool to edit low poly models. Has good support for Half-Life.
MilkShape 3D - A popular (but non-free) model editor with good support for Half-Life. A 30 day trial is available.
Blender - A free, fully-featured modelling program that supports animations. Install the Blender Source Tools (which also support Goldsource) to get Half-Life support. A guide is available here.
Crowbar - A convenient app which allows you to easily compile and decompile GoldSource and Source models.
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.
Half-Life SDK on GitHub - Official GitHub repository for the same code released in the SDK, but with additional updates and bugfixes.
Half-Life SDK 2.3 - Though heavily outdated, this package encompasses everything Valve made available to the general public back in the day, including model and map example source files, early models and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.
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.
Solokiller's HLMV - Updated 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.
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.
GS HL Tools - TWHL user Stojke maintains this repository of useful Goldsource-related modding tools.
TWHL, of course! - The site you're on right now! We have forums and various other things. Be sure to make use of all our resources!
Sven Manor - A resource site focus on Sven Co-op editing, but much of the information can be used for any Goldsource editing. The Sven Co-op forums are a good place for general Goldsource discussion as well.
The303's Tutorials - An excellent collection of Goldsource tutorials with a focus on textures, sprites, and modelling.