Source SDK is described as 'The Source SDK is a software development kit compiled by Valve Software that is used to create maps or mods for the Source engine, with the exception of Left 4 Dead, Left 4 Dead 2, and Alien Swarm' and is an app in the Development category. There are nine alternatives to Source SDK for a variety of platforms, including Windows, Linux, Mac, Android and iPhone. The best alternative is Unreal Engine, which is free. Other great apps like Source SDK are Stride Game Engine (Free, Open Source), Urho3D (Free, Open Source), Open 3D Engine (Free, Open Source) and SmartShape (Free, Open Source).
Unreal Engine is a professional suite of tools and technologies used for building high-quality games across a range of platforms. Unreal Engine’s rendering architecture enables developers to achieve stunning visuals and also scale elegantly to lower-end systems.
Stride (formerly Xenko) is an open-source C# game engine designed for the future of gaming. It comes with a full toolchain and is especially well suited to create realistic games but allows you much more!.
Open 3D Engine (O3DE) is an Apache 2.0-licensed multi-platform 3D engine that enables developers and content creators to build AAA games, cinema-quality 3D worlds, and high-fidelity simulations without any fees or commercial obligations.
Your project’s smart 3D digital twin as a Service. SmartShape (formerly Minko) is a technology to develop and design rich, interactive and universal 3D applications targeting computers, web browsers and mobile platforms.
Maratis is a portable, simple and visual game development tool designed for artists and developers. The Engine has been tested on Windows, Mac, Linux and iOS and can be used virtually on any platform. Maratis is Free and Open-source.
UnrealEd (UEd for short) is the level editor used to create levels for Unreal engine games (eg UT2004 or Killing Floor). Built-in scripting language called UnrealScript allows for editors to customizing game content.