A list of various other software that is useful for mapping for the Half-Life engine.
An open-source map editor, which is still kept up to date and has many useful features. Also supports many different games on different engines.
The official map editor by Valve. The latest version of the Valve Hammer Editor version is version 3.5 beta (linked-to above) - there is also an older stable version 3.4.
This editor is no longer actively developed.
Forum with lots of software, some relatively recently updated.
Many different tools useful for mapping. Includes Batch Compiler, GCFScape (lets you open Steam's .gcf files), and Terrain Generator (for making natural-looking terrain).
Optimizes the entities in compiled maps, allowing you to include more entities and also reduce network traffic.
Download: version 1.1.1 (stable) version 1.1.2-dev (development version)
Allows you to view Half-Life 1 models, including their animations. This is an updated version of the original Half-Life Model Viewer.
A freeware texture editing program.