If you simply cannot wait until "S.T.A.L.K.E.R. Shadow of Chernobyl" Russian multiplayer beta demo has been released, giving players a chance to try out the anticipated FPS by GSC Game World.

"The Zone exploded, radioactive clouds covered vast territories. A dreadful amount of people, animals and forests perished. Thirty kilometres of the area got cordoned off by the army, scientists failed to explain what had happened. The Zone grew, various death-bearing anomalies were spotted inside. Some invisible force tore living beings apart, inflicted awful blood-sputtering wounds. Expeditions faced mutant animals, unlikely to have appeared in the Zone through natural course of evolution. The catastrophe, mutants, anomalies, contamination� Everything appeared to be a consequence of some eerie phenomenon�"

The 430MB demo can be downloaded from the GSC FTP, FileFront, or Worthplaying.

Update 2: It looks like you are able to do LAN games but you need a key for internet multiplayer.