https://github.com/ValveSoftware/halflife/blob/master/README.md

You may, free of charge, download and use the SDK to develop a modified Valve game running on the Half-Life engine. You may distribute your modified Valve game in source and object code form, but only for free. Terms of use for Valve games are found in the Steam Subscriber Agreement located here: http://store.steampowered.com/subscriber_agreement/

You may copy, modify, and distribute the SDK and any modifications you make to the SDK in source and object code form, but only for free. Any distribution of this SDK must include this license.txt and third_party_licenses.txt.

DISCLAIMER OF WARRANTIES. THE SOURCE SDK AND ANY OTHER MATERIAL DOWNLOADED BY LICENSEE IS PROVIDED “AS IS”. VALVE AND ITS SUPPLIERS DISCLAIM ALL WARRANTIES WITH RESPECT TO THE SDK, EITHER EXPRESS OR IMPLIED, INCLUDING, BUT NOT LIMITED TO, IMPLIED WARRANTIES OF MERCHANTABILITY, NON-INFRINGEMENT, TITLE AND FITNESS FOR A PARTICULAR PURPOSE.

LIMITATION OF LIABILITY. IN NO EVENT SHALL VALVE OR ITS SUPPLIERS BE LIABLE FOR ANY SPECIAL, INCIDENTAL, INDIRECT, OR CONSEQUENTIAL DAMAGES WHATSOEVER (INCLUDING, WITHOUT LIMITATION, DAMAGES FOR LOSS OF BUSINESS PROFITS, BUSINESS INTERRUPTION, LOSS OF BUSINESS INFORMATION, OR ANY OTHER PECUNIARY LOSS) ARISING OUT OF THE USE OF OR INABILITY TO USE THE ENGINE AND/OR THE SDK, EVEN IF VALVE HAS BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES.

All artwork, sounds, audio, screenshots, text and code in Natural Selection, Zen of Sudoku, Spark engine and Natural Selection 2 are Copyright © 2014 Unknown Worlds Entertainment, Inc (http://www.unknownworlds.com).

The mark Natural Selection was first represented in association with video-game software in June of 2001, and was first used in commerce around January, 2002. Natural Selection is Registered with the U.S. Patent and Trademark Office (No. 4,179,393).

See COPYING.txt for the GNU GENERAL PUBLIC LICENSE

EXCLUDED CODE: The code described below and contained in the Natural Selection Source Code release is not part of the Program covered by the GPL and is expressly excluded from its terms. You are solely responsible for obtaining from the copyright holder a license for such code and complying with the applicable license terms.

EXCLUDED CODE AND LIBRARIES

• Half Life 1 SDK LICENSE (Copyright(C) Valve Corp.)
• FMOD 3.7.5
• LUA 5.0 (http://lua.org)
• Particle system library by David McAllister (http://www.cs.unc.edu/techreports/00-007.pdf).

Original code and design by Charlie Cleveland ([email protected], @flayra).

Many contributions from Karl Patrick ([email protected]), Petter Rønningen [email protected], Harry Walsh [email protected], and probably lots of people I forgot.