S.T.A.L.K.E.R.: Lost Alpha is a mod for S.T.A.L.K.E.R.: Shadow of Chernobyl. However, since so much have been changed, it might be released as a stand-alone game.
The mod recently won 4th place over at Moddb's 2011 Mod of the Year Awards (http://www.dezowave.com/lost-alpha-4...best-upcoming/).
Vintar, a periodic visitor to this forum, is on the Lost Alpha development team and will hopefully keep us (read: me and axon1988) updated on the project.
More details here: