Team Fortress (Quake)
Where it all began. The teams are there, as are the classes - but there’s none of the colour seen in Valve’s eventual standalone version. The original Team Fortress does support up to four teams in Capture and Hold, Hunted and Capture the Flag, however - and active communities remain around server-side mods like Custom TF and MegaTF.
Zombie Panic: Source (Half-Life 2)
A now-familiar living dead scenario is enlivened by an ingenious ruleset: one player volunteers to be the first zombie, and the rest stay alive long enough to complete their objectives. Killed human players rejoin the game as undead and pursue their former teammates - and the survivors are granted extra respawns as their numbers shrink.
Mental Omega (Command & Conquer: Red Alert 2 - Yuri’s Revenge)
Beginning as a balance mod but growing into an unofficial expansion pack, Mental Omega offers new campaigns for Red Alert’s alternate-universe factions: the Allies, the Soviets, and psychic megalomaniac Yuri’s Epsilon. The first of two intended single-player Acts features a new take on the events of Red Alert 2.
iCEnhancer (GTA IV)
GTA V on PC is pretty, but nobody at Rockstar North has come so close to climbing out the uncanny valley as Hayssam Keilany. The New Zealander’s iCEnhancer builds on graphics mod ENB and shader suite SweetFX to beef up Liberty City’s textures, lighting, shadows, colours, reflections, bloom and anti-aliasing to a state of near-photorealistic noir.
Portal: Prelude (Portal)
A series of puzzles that tell the origin story of GlaDOS - in this version of events an AI built to take over the tedious, lengthy and repetitive tasks of Aperture’s testing staff. At the opening of Prelude, the ethically-challenged company’s employees are eagerly awaiting their replacement. At the end - well, expect things to have changed.
They Hunger (Half-Life)
Released in three episodes between 1999 and 2001, They Hunger was (and still is) a Mary Shelley-inspired story that picks up the body-horror of Half-Life’s headcrabs and runs with it - spinning a tale of an asylum-cum-lab and the threat its undead monsters pose to the surrounding town of Rockwell.
Star Trek: Armada III (Sins of a Solar Empire: Rebellion)
The spiritual successor to the Activision-published RTS that found the Borg threatening the Alpha Quadrant, Armada III brings not only the Borg but also the Klingons, Romulans, Federation, Cardassians, Dominion and Breen. Picard makes an appearance, and music is sourced and remastered from other Trek games. A bold enterprise.
Insurgency (Half-Life 2)
So successful it spawned a standalone game on Steam, Insurgency still exists as a free total conversion - you only need one Source-based game and a Steam account to play. It’s a modern military shooter that prizes good organisation above all else, introducing specified squad roles, morale and a chain of command.
Natural Selection (Half-Life)
Another wildly successful multiplayer mod that eventually outgrew the pot of its parent game, Natural Selection is a hybrid FPS/RTS that pits Marines against the Kharaa in large spaceships and stations. One member of the marine team enters the ‘command chair’ - purchasing upgrades, dropping supplies, placing buildings and issuing orders - which players are expected to follow.
A Clash of Kings (Mount & Blade: Warband)
Adds sea travel, naval battles, props, textures and over a thousand new items, and converts Mount & Blade’s medieval trappings into equivalents from George Ah-Rah’s world. Many characters from the A Song of Ice and Fire novels have turned up to scrap for a corner of this massive map of Westeros.