I've uploaded gameplay of Shadow of Chernobyl over on my gaming channel today. That can be found by clicking here.
Shadow of Chernobyl was released in mid 2007 by GSC Game World.
"S.T.A.L.K.E.R. takes place in an area called "The Zone", which is based on the real-life Chernobyl Exclusion Zone and partly on the settings of the source novel and film. It encompasses roughly 30 square kilometers and features a slice of Chernobyl extending south from Chernobyl Nuclear Power Plant; geographical changes for artistic license include moving the city of Pripyat into this area (it is actually to the north-west of the power station), although the city itself is directly modeled on its real-life counterpart, albeit smaller in size." (Wiki)
This is one I would definitely recommend to anyone who is into FPS and the whole survival horror games. You can purchase this one on your steam account for $20 and it's awesome. There are two others Call of Pripyat and Clear Sky. If I had to choose I'd choose Clear Sky (more roaming and freedom).
But as I said before this game I hadn't seen one that you could roam around and accomplish missions (sounds like Borderlands).
The few elements of realism that they did have in this game where - hunger, bleeding, radiation poisoning, weather, and day time. In this game you had to carry around food so you didn't die of hunger. I believe in the second S.T.A.L.K.E.R (Call of Pripyat) you had to carry water as well. But you also had bandages to stop bleeding, and radiation could be neutralized by wearing artifacts and using medication. Also, you had just about all the weather except for snow which was amazing. And finally, the setting did fluctuate from night to day with an easy transition.
I will be posting sources for mods - this game you can only carry a limited amount of gear *60* and limited ammo so I am going to be modding this to make it more enjoyable.