The most famous one is one called The Hall of Tortured Souls.
It works in old versions of Excel ('95).
1. Open a new file.
2. Scroll down until you see row 95.
3. Click on the row 95 button, this highlights the whole row
4. Press tab, to move to the second column.
5. Now, move your mouse and click on help, THEN about Microsoft Excel
6. Press ctrl-alt-shift and click on the tech support button simultaneously.
7. A WINDOW WILL APPEAR, TITLE: THE HALL OF TORTURED SOULS you will see something freaky! it has a doom style format and you can walk all around the hall (using your arrow keys)...and on the sides of the walls are the names of the tortured souls....
8. NOW WALK UP THE STAIRS AND THEN COME BACK DOWN, FACE THE BLANK WALL AND THEN TYPE IN EXCELKFA.
This will open the blank wall to reveal another secret passage, walk through the passage and DO NOT fall off (this is the hard part!), when you get to the end, you will see something really, really eerie.... (This is REALLY there! I haven't been able to reach the end of this VERY narrow passage though.... - WILANT) Since you likely don't have the old excel here are some pictures of this weird backdoor: