S.T.A.L.K.E.R.: Shadow of Chernobyl Easter Egg - Half-life 2 Reference.

In some part during the game, you can find what might just be Gordon Freemans PDA, it mentions Black Mesa and a strange town in Russia, where City 17 is located at. And in the second entry in the PDA, he saids he had to give his crowbar away for a can of meat.