Hammertime

Hammertime is an open-source Fork of Sledge which is an open-source Alternative to Valve's Hammer Editor for the Goldsource engine. It is written in .NET and uses Veldrid for rendering, with DirectX 10.

Hammertime supports Goldsource mapping, and is constantly being updated to add more Features, some of these Features include:

• Prefab Support
• (Additional Features WIP))

Here are also some good Community Editors if you need some:

• TrenchBroom (as of writing, TrenchBroom has some small Support for Half-Life, but is dependent on Community Contributions to expand the Support for Half Life, so its not complete right now.)
• Valve Hammer Editor (Found in the steam Library which you get when you own Half Life 1)
• JACK (the free version, don't pay for it, since it isnt being updated by the developer.)

The Hammertime Editor is licensed under BSD-3.