Superficial Intelligence

These may classify more as cheats than easter eggs, but they're still undocumented and fun to try.

In Windows Solitaire, you can press ALT + SHIFT + 2 to immediately win the game. Similarly, in Freecell, you can press CTRL + SHIFT + F10. A dialog will pop up telling you to abort to win, retry to lose, or ignore to cancel. With this cheat, you can make sure to lose every time if you want! Freecell also has two secret, and I believe impossible, games. In the select game option, type in "-1" or "-2" to try these games out.

Minesweeper also has a very neat cheat, although I'm not sure if it works on every system (works on mine!). When you open Minesweeper, just type "xyzzy" and then hold SHIFT for at least three seconds. Afterwards, whenever you hover the mouse over any of the tiles, the pixel on the upper-left corner of the screen will change to white if there is not a mine and to black if there is. Great way to impress your friends (at least those who are nerdy enough to be impressed by such an act)!