The Orange Box Easter Egg - The Hev Panel Returns.

Start the mission Our Mutual Fiend. Play through the level and seal the second silo. Follow Alex and find Eli watching the transmission from Mossman. When you enter the room to your right there should be a fenced off area. Jump on the couch next to it and get out your Gravity Gun. Inside there should be a wooden box. Break it and you will see a yellow item. Grab it and pull it out and you will see that it is the Hev panel from the first Half-Life. Plus it is the same one from the second game.