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. There's been some wonderful projects that have given us complete rebuilds as well as addons for the three campaigns. 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.
http://www.moddb.com/mods/translations
https://www.moddb.com/mods/sigerous-sgm

More recently some of the best modders have teamed up to create there own visions of the Zone.
There are two major projects that come to mind. For the original Stalker Shadow of Chernobyl game there's Lost Alpha, The LA developers have now released the Directors cut which is a must play standalone Stalker build, http://www.moddb.com/mods/lost-alpha

For the final game of the trilogy Stalker Call of Pripyat there's a stand alone modification called Stalker Call of Chernobyl, Since it's initial release the team behind the mod have been very active taking feedback and suggestions onboard as they've fine tuned their work. Call of Pripyat was set across 5 maps, none of which were in the earlier games. Call of Chernobyl contains those 5 as well as all the maps from the first two games, GSC's unused build maps & a fan made map.

Call of Pripyat had a freeplay option where at the end of the campaign you can choose to remain in the zone to roam around as you please, unfortunately there was not a lot to do apart from mutant and artifact hunting. Call of Chernobyl is a freeplay mod which solves this problem by providing a vast open world with a variety of tasks and missions that are spread across the Zone. There's also a story mode as well as the option to start your game as a member of any faction you choose, all of which have there own friends and foes meaning the Zone is not your only adversary.

With COC you're getting what is as good as a brand new addition to the S.T.A.L.K.E.R. series, A fourth game that is available via the modding site Moddb so give it a go Stalker you won't be disappointed. http://www.moddb.com/mods/call-of-chernobyl

When Stalker 2 was cancelled it looked like it was the end of the road for STALKER but thanks to the modding community we continue to get new content & new reasons to enter the Zone making it both bigger and badder than ever before,
Best of all mods are free,