The Stalker series in general has many different and great mods.
There's the Zone Reclamation Project which fixes some bugs and enhances graphics for the original game. In addition there are options to customize things like how much you can carry, whether or not enemies use grenades, whether or not you can have vendors use weapons ect. All of which without drastically changing the original game. Pretty great for newcomers or those who just want to re-experience the original game again.
Autumn Aurora completely overhauls the look and feel of the Zone.
Misery for Call of Pripyat enforces a much more survival based experience.
And of course there's Call of Chernobyl. A giant ass sandbox featuring all the locations for all three of the games which lets you create your own character and which faction he's from. It even has an Ironman mode if you're hardcore like that. And even an interesting mode where when you die, you gain control of a random NPC somewhere else in the Zone and play as him.