>>15646837
Doom 3 had the dev room with the PDA, which was neat.
Yoshi's Island had an enemy you could kill that would give you a message box, which had a 2-player minigame code.
Fallout Tactics had enouncters that could actually give you useful stuff.
Unreal and Unreal Tournament had hidden rooms, like the "PRETTY FLY FOR A WHITE GUY!" texture in the slow-kill room.
Halo Reach did one I really like, which is they created a room you have to glitch into that had a bunch of concept art and memos. Dev easter eggs with cut content and side commentary is always the best.