Due to popular demand, GSC Game World has released an old version of S.T.A.L.K.E.R.: Shadow of Chernobyl - 'xrCore' build 1935, Oct 18 2004.
Originally announced in November 2001 for a 2003 release, S.T.A.L.K.E.R.: Shadow of Chernobyl (or S.T.A.L.K.E.R.: Oblivion Lost as it was known then) was renowned for its long development cycle and frequent delays.
GSC Game World has released a build from 2004 which utilised a completely different physics engine than the one used in the final retail version. This build is fully playable and contains all of the original levels as well as those that were cut such as Dead City, Swamp and Generators.