Where many developers now charge users for in-house total conversions (Battlefield Bad Company 2: Vietnam, for example) PC gamers have long been able to enjoy similar efforts for free. Many of these games survived the jump from the original Half-Life engine to the Source engine, first used in Counter-Strike: Source & Half-Life 2 (and re-used several times in games like Left 4 Dead). Thanks to the free software development kit (SDK) modders, mappers and modelers were free to hack and mould the engine into completely different games. Valve's original hit first person shooter Half-Life gave birth to some wonderful third-party development.
