The mod is one of PC gaming's greatest achievements. You buy a game, play it to death, and then chances are passion-driven fanatics will release non-commercial content for us to devour. It can extend the life a title for years.
Naturally not all projects born out of a love of the subject matter impress, but over the years we have seen numerous gems emerge from the mod community. We've been poring over our murky mod-playing past - and slightly less murky present - and selected 12 we'd buy a beer if we bumped into them down the pub...
Mod for: Battlefield 1942
Official website: www.desertcombat.com/
It's testament to its quality and the regard in which Battlefield 1942 mod Desert Combat was held that members of the mod team are now in the full time employ of THQ and working on Frontlines: Fuel of War.
Although it didn't add anything fundamentally new to the core Battlefield 1942 gameplay, what it did do is take us out of World War II and onto 21st century battlefields and present us with an extremely professional product. Eventually we were of course to see the Battlefield series embrace the same modern-day setting.
Mod for: Unreal Tournament 2004
Official website: www.blackcatgames.com/swarm/
Black Cat Games chose to take Epic's Unreal engine and created a top-down, sci-fi shooter that was at times bastard hard but excellent nevertheless. Playable single-player or in co-op, Alien Swarm in a nutshell pitted a group of marines against alien hordes, with battles occurring on spaceships.
Four unique classes added plenty of flavour to the action, and the real cream on the cake was Alien Swarm's ability to conjure up images of the movie Aliens while you were blasting away like a loon. Fantastic. Black Cat Games is currently working on the follow-up Alien Swarm: Infested, which uses Valve's Source engine and will be released as a commercial product.
Mod for: Half-Life 2
Official website: www.hylobatidae.org/minerva/
Arguably the best Half-Life 2 single-player mods out there - and one of the best single-player mods full stop - Minerva: Metastasis combines action with enigma to fantastic effect. It's been split into four chapters and the third and fourth aren't far short of being of a level of quality you'd normally expect to have to cough up for.
Anyone who's played it will know it can be intriguingly weird, but essentially you strap on boots as a mystery protagonist who is being directed by an entity named Minerva in the fight against the Combine. The decision to have Minerva communicate in text form was a stroke of genius, even if she/they/it does mix baffling riddles and indirect hints into the communications.
Ultima V: Lazarus
Mod for: Dungeon Siege
Official website: lazarus.planetdungeonsiege.gamespy.com/
Truly the amount of work and love that has gone into Ultima V: Lazarus is mind boggling, and it's hardly surprising that the folks behind it - Team Lazarus - describe it as an "exceedingly ambitious project".