Half-Life Cheats

During the mission 'On A Rail' (the part of the game where you have to stop the rockets from firing and your in the sewers or something on a trolley). Turn on 'noclip' and float towards the top of the rocket. once you get to the top you can see a pic of a developer of the game. its his face tiled about 50 times on a square. there is a ladder at the top that brings you up another level with the same pic.

P.S - this Easter Egg is only reachable using 'noclip'. there is no other way to get up that high