See Source SDK 2013 for more information. This time around, the source code is not made available via an SDK launcher, but is instead hosted on Github. With this change, they updated the source code to the latest version of the Source-1-Engine for the HL2-based engine branch. In 2013, Valve phased out the Source SDK with the release of SteamPipe. It also lacks many of the HL2-compatible code's peripheral tools (including custom shader support) and cannot access HL2's content.

Alien Swarm is totally free to play and its SDK includes source code, but it's source code for a top-down shooter. However, you cannot make a mod for Source 2009 you can make a content-only mod of a 2009 game by following Creating a Mod manually. This is the current version of the engine with Mac support. This is the most recent HL2-Based engine branch available. Only included for backwards compatibility reasons.

Outdated - Original Half Life 2: Episode One mod. Change the SDK's engine version to the one you want. Before starting a mod you need to choose which version of Source you will use.

Early versions of half life 2 download