There are two major projects that come to mind. More recently some of the best modders have teamed up to create there own visions of the Zone.

I've made English translations for what I think are some of the best Russian mods and then uploaded pre built versions that can be found on Moddb. There's been some wonderful projects that have given us complete rebuilds as well as addons for the three campaigns. Many years back the company behind the Stalker series (GSC) made their in house tools & assets for the games available to PC gamers so they can create mods for the 3 Stalker titles, Today the games have something of a cult status with large communities that continue to play & mod the STALKER games, There's been some amazing mods over the years and fan made assets are freely shared among the community.