http://kotaku.com/5685349/how-to-unlock-zork-and-call-of-duty-black-ops-other-hidden-games
Got a copy of Black Ops? Then you've got the first Zork game. Follow along with Kotaku's video, and you, too, can play Zork on any of the current-gen platforms (excluding the Wii)!

Games within games are awesome.

First example of this I remember is Maniac Mansion game in Day of the Tentacle. Both are nice games.