Half-Life and Portal creator Valve will release a software development kit for its proprietary Source Engine and modding tools free of charge.Normally, the Source SDK is only available to those who purchase a game built on the Source Engine. However, now that Team Fortress 2 has gone free-to-play , the SDK has too.In addition to offering users free development tools, the release also gives users access to a variety of free mods.Currently, the freely-available SDK appears to be more of a side effect than a planned initiative, as only certain mods work with the SDK provided, but Valve plans a more formal roll-out soon."We are in the process of getting it all done," Valve's Robin Walker told blog Rock, Paper, Shotgun. "It's a bit messy because we have multiple versions of the SDK, and there's some dependencies we need to shake out. But yes, the gist of it is that we're just going to go ahead and make the Source SDK freely available."More information on the Source SDK is available on its official website