It would be selling Stalker: Anomaly short to call it a mod. Based on Stalker: Call Of Pripyat, GSC Game World's haunted Ukranian survival sandbox, Anomaly feels more like a sequel than a user-made collaboration. Built on the now open-source and community-maintained X-Ray Engine, it features the combined and overhauled environments of all three official games in the series (plus some new ones) making for a sprawling, unpredictable sandbox full of mutants, mercenaries and murder.
More surprisingly, from the dozen hours I've played so far, it seems far more stable and polished than any official game in the series. Impressive, considering that it looks and runs better, too.
Anomaly has been kicking around for a while, but this week Version 1.5.0 was released, effectively a re-launch, overhauling huge swathes of the game and adding tons of new content and features. There's a deep and involved crafting and maintenance system, a mask system inspired by the Metro series and a reworked, far more polished UI. On top of the procedurally generated missions and faction quests, there are now four main story arcs to follow, several leading you to new factions trying to exert their influence on the Zone.
If the story mode doesn't appeal, then you can play in Warzone mode instead. Without the need to keep story NPCs alive, the AI is free go wild. Every faction is now vying for control of the Zone, and you can become commander of one of the factions, leading your forces across the map, RTS style.
Or maybe if you prefer a more solitary, dangerous life, Azazel mode will appeal. A fun take on Ironman that puts you in control of a random NPC somewhere else in the world upon dying. You might even find your old body (or whoever looted it after killing you), or perhaps you'll just get eaten by roaming mutant cats. Truly the Dark Souls of Stalker modes.
Be warned, though: Anomaly is a tougher, less forgiving sandbox than the original game, with a greater focus on survival and simulation. You need to eat, drink and sleep, although it's easy enough to keep those gauges in line, so long as you're making a bit of money from random missions. Read the manual, keep your PDA batteries charged, and good hunting.
You can find Stalker: Anomaly on ModDB. It's entirely standalone (no purchase needed), and takes up a hefty 19.1gb of hard drive space when unpacked. If ModDB's servers are creaking, then it's mirrored here on Google Drive.