Crysis is an upcoming science-fiction first-person shooter game that is currently in development by German developer Crytek for the PC only.

Ever since I first layed my eyes on this game at E3, I knew this was going to be something big. This game has by far the best graphics I've ever seen in-game, which is why it's just a PC release. It's also Vista only, as the game requires Direct x10 which is only available for Vista users. You'd also need one beast of a rig to run this.

Set in 2019, you play as Jake Dunn, a United States Delta Force operator. Dunn and his team are sent to pacify a situation in which a group of United States archaeologists are taken captive by the North Korean military while investigating a mysterious ancient artifact that was found in the Spratly Islands, located in the South China Sea.

Here are some videos of Crysis.
Trailer.
Gameplay Footage.
Demonstration of the game engine. (CryEngine 2)
Definitely watch the last video if you want to see the amount of realism that is involved with the game.
Now, this game, like many others, will be very customizable and it will release with an SDK, which means you'll be able to map and mod for it. There's a team working on a mod already based on the '28 Days/Weeks Later' movies. It's called 28 Months Later, as you'd imagine.

The Scene
It started almost 2 and a half years ago, the Rage virus spreads from research labs in London UK. The virus can turn a healthy man or woman into a blood thirsty zombie within 20 seconds. In under a week nearly the whole of Britain was infected seeking the living to feed off. After about 7 months it was believed that all the zombies had starved to death. London started letting people in from overseas, to help repopulate and clean up what was once a great nation.
Under the watchful eye of the United States Army life started to return to normality in London, though civilians could never leave the city without a military escort. What happened next is unclear; some how from what we have gathered the virus got back into London and it started to spread fast. Soon enough the US military were ordered to kill everyone. Snipers on the roofs at first were hesitant and only shot infected, but it soon became clear that everyone needed to be killed. After a swift extraction of the US troops London was attacked from the air by US jets dropping napalm on the streets. Thousands of the infected and uninfected were burnt to death. Many survivors had made it into the underground rail network and fled the hordes of infected zombies. We heard that the military were in the tunnels waiting with flame throwers and heavy machine guns, but we don’t think that was enough.
Five hours later we got the news we had all been dreading the virus had escaped the British Isles and was now spreading its way through France and Western Europe. On hearing this world leaders executed their fall back plans maximising security placing towns in high quarantine, with large military defences. Tragically within a few months most of the towns were over run by the infected. The world’s population was reduced from 6 Billion to an estimated hundred thousand within a year. Rations are running out, the long wave radio broadcast says its over, people start emerging from their shelters unaware that the virus is still at large, now being spread through vermin. It’s only a matter of time before the infection gets the humans again, where will you stand? Will you run and hide, or will you fight? Either way you must survive.
The Mod
28 Months later will be a total conversion for Crysis set in this apocalyptic scenario. The mod will be played online with up to 32 players. The aim of our team is to produce a game that will not only scare the pants off you but it will also make you think. We will be utilising the fantastic physics engine to its fullest extent allowing players to use any objects they find in the world as weapons or barricades.
The main game mode we will be focusing on is called “infection”; it starts off with one player being infected his aim is to infect all the other players before the end of the round. To infect the living they must be bitten by a zombie. If a zombie kills a human in another way (say he throws an explosive barrel and kills someone) that player will loose some points but he will respawn as a Human. If a zombie gets killed by anything he gets a choice; one he can respawn at the set starting positions, or two he can wait 20 seconds and get back up where he fell. However the humans can prevent this by destroying the zombies head, forcing the zombies to have to spawn somewhere else. When a human gets bitten, there will be a 20 second gap before he becomes a zombie, so other humans may use this time to kill him before he becomes a threat. After 20 seconds that person will now be on the zombie team and will now get points by infecting the humans.
We plan on making 2 human factions, Military and Civilian. It will depend on what map is being played as to what faction features. Military will have much greater weaponry and vehicles, but they are limited on ammo and fuel. Civilians will have to depend on more natural weapons, just regular items that you can find around the maps. The zombies will have a choice of class for when they spawn. You get your heavy zombie; they’re slow but very strong, capable of throwing large objects and tearing apart small barricades, they can also take more damage. Light zombies will be fast and agile capable of jumping over things most people cant. However they can’t take as much damage as the heavy zombies. We are still deciding on other classes.
http://www.28-months-later.com
