The 19 NES games in Animal Crossing, the TMNT arcade games in the TMNT 2 and 3 GCN/Xbox/PS2 games, the MSX Metal Gears in MGS3 Subsistence, the arcade games in Pac-Man 2, 2600 Pitfall in The Mayan Adventure, DK arcade and Jetpac in DK64 (even on Wii U!), the games in the SNES Beavis and Butt-Head.
Is there a complete list somewhere of all hidden games, even unlockable extra games in stuff like Namco Museum and SNK Arcade Classics?