Valve's original hit first person shooter Half-Life gave birth to some wonderful third-party development. Thanks to the free software development kit (SDK) modders, mappers and modelers were free to hack and mould the engine into completely different games.
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). 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.
Here are a few personal free favourites to get the most out of your Source engine games!
Full Steam Ahead
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).
You're going to need a copy of a game that uses the Source engine and supports third-party mods. If you've got a full copy of Half-Life 2 then you're already done. 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.
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.
Zombie Panic! Source [On Steam]
At the time of writing Zombie Panic! Source is entertaining more online players than a considerable amount of new full-price releases, which is pretty good going for a free mod. There are two classes - the human survivor and blood-thirsty zombies.
Everyone starts out human, apart from one (volunteer or auto-selected) zombie. As a human it's your job to fend off hordes of undead with whatever weapons you can find (you're given one random melee weapon and a handgun to start you off). Zombies simply have to eat brains!
When a human player is killed by a zombie then he or she joins the zombie team. Each zombie has a limited amount of lives, and when these lives run out the humans win. Of course, if the zombies overrun the human team then the zombie team wins.
Fortress Forever [On Steam]
Do you remember Team Fortress Classic? Back when mods were free add-ons, created by enthusiasts in their bedrooms and played by thousands? Don't you wish someone would remake the classic TFC, except from scratch on the Source engine? Step in Fortress Forever!
The mod aims to satiate the needs of those who have stuck with the age-old TFC, and also promises to have done "more for new TF players than any prior Fortress attempt" - which should please the newbies.
As well as the old school capture the flag set-up, FF includes several other game modes, 10 playable classes and in excess of 20 maps (including the legendary 2fort). If you pine for TFC and are sick of Team Fortress 2, you might have just found your medicine.
Insurgency: Modern Infantry Combat [On Steam]
Winner of ModDB's Mod of the Year 2007, Insurgency: Modern Infantry Combat delivers a present-day conflict experience with a variety of real-world weapons and equipment. You won't get very far without working as a team and making the most of the environment provided.
The realism levels have been turned up to 11, meaning a couple of bullets is all it takes to be pushing up daisies. There's a few modes of play, including a Battlefield-esque point capture gametype and of course generic team deathmatch.
Maps are based on real-world Afghan and Iraqi locations such as Ramadi, Almaden and Baghdad. There is also a new (unrelated) open source Insurgency Classic project, which might be worth checking out.
Source Forts [On Steam]
Recently back from the dead, Source Forts is a multiplayer mod with a difference, and will test your building and shooting skills equally. Armed with a modified Half-Life 2 gravity gun, teams must create and maintain their very own forts in order to protect their flag.
At the start of each round there is a build period, where you and your team are given enough time to construct a base. At the same time your foes are busily doing the same. Once build-time is over, you must capture the opposing force's flag more times than they capture yours.
There are different classes, plenty of maps and a small dedicated community playing. If you're interested, you'll want to check out the wiki [Broken Link Removed] for the full low-down.
Conclusion
Whilst there might not be a wealth of players enjoying these mods, there should still be enough available servers to get a decent game. If you're looking for a neat way to link up with and meet gamers try X-Fire for Windows, a game-centric IM client.
Unfortunately for gamers these mods seem to be thinner on the ground than ever - despite the original (and free) Half-Life classic Counter-Strike still being one of the most popular games on the Steam platform.
Do you have any favourite free mods? Do you remember the good old days of Counter-Strike beta 7? Are free mods soon going to be a thing of the past? Share your thoughts in the comments below.