
The issue is that thanks to SteamPipe, the locations of these files have changed. Gameinfo.txt tells Steam all the content that needs to be loaded. Navigate to the mod folder and open the gameinfo.txt file. If you’re not in luck, this is where things can get tricky. Now load up Steam and try it out, as some mods such as those using an SDK Base and new releases may be unaffected.

Luckily there is a list of games that support these mods on the Steam website, choose one you like the look of and make sure it's downloaded and ready to play.The first step is to download and install the mod as usual, placing the folder in the Steam\SteamApps\sourcemods directory. If you've not got Half-Life 2 you could buy it (it's about $10, £7) or a similar title with support for the SDK.

If you've got a full copy of Half-Life 2 then you're already done. You're going to need a copy of a game that uses the Source engine and supports third-party mods. If you've not got Steam, head over to the official website and download the client for Windows (Steam does work on Mac, as does Half-Life 2 but unfortunately free third-party mods that run on OS X are very thin on the ground). Here are a few personal free favourites to get the most out of your Source engine games!

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.
