Game: Stalker: Call of Pripyat
All those weapons scattered around? Gone. Anomalies? Now more dangerous. Magic mini-map? Forget it. Valuable quest rewards? Good luck. Things you do get: thirsty, and factions who send goons after you if you anger them. On the plus side Pripyat is much more active, with a complete sound overhaul, and new NPCs to meet—who all have to play by the rules too, with no more infinite ammo. If you can survive here, you've got a good chance when the actual apocalypse comes.
Fallout: New Vegas
Link: Nexus Mods
Nevada is a good example of making things more difficult without being openly psychotic. Levelling is slower, players and NPCs get less health, and obvious features are now in, such as armour only being a factor in headshots if the target actually has head protection. It's also possible to toggle some extra-hardcore options, such as food no longer healing and taking care of hunger/thirst/ sleep on the move. There's a sack of new content, and an Extra Options mod is also available, offering even more control.
Despite what modern 'old-school' shooters would have you think, Doom was a relatively sedate experience—fast running speed, yes, but lots of skulking in the dark and going slow. Not any more! Brutal Doom cranks everything up to 11, then yawns and goes right for 25.6. We're talking extra shrapnel, execution attacks, tougher and faster monsters, metal music, and blood, blood, blood as far as your exploding eyes can see. It's compatible with just about any level you can throw at it, turning even E1M1 into charnel house devastation. The enemies don't get it all their own way, as Doomguy now starts with an assault rifle rather than simply a pistol, and a whole arsenal of new guns has been added to the Doom collection—including the BFG's big brother.