Links to various other software that is useful for mapping for the Half-Life engine.
The official map editor by Valve. The above link is for the latest stable (non-beta) version. Valve have also released a Hammer Editor 3.5 beta version.
This editor is no longer actively developed.
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.
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.
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.