Jensen Harris's blog post today talked about an early Easter Egg he found in the Radio Shack TRS-80 Color Computer BASIC interpreter.
What's not widely known is that there were Easter Eggs in MS-DOS. Not many, but some did slip in. The earliest one I know of was one in the MS-DOS "Recover" command.
The "Recover" command was an "interesting" command.
As it was explained to me, when Microsoft added support for hard disks (and added a hierarchical filesystem to the operating system), the powers that be were worried that people would "lose" their files (by forgetting where they put them).
The "recover" command was an attempt to solve this. Of course it "solved" the problem by using the "Take a chainsaw to carve the Sunday roast" technique.
You see, the "Recover" command flattened your hard disk - it moved all the files from all the subdirectories on your hard disk into the root directory. And it renamed them to be FILE0001.REC to FILE<n>.REC.
If someone ever used it, their immediate reaction was "Why on earth did those idiots put such a useless tool in the OS, now I've got got to figure out which of these files is my file, and I need to put all my files back where they came from". Fortunately Microsoft finally removed it from the OS in the MS-DOS 5.0 timeframe.
Before it flattened your hard disk, it helpfully asked you if you wanted to continue (Y/N)?.
Here's the Easter Egg: On MS-DOS 2.0 (only), if you hit "CTRL-R" at the Y/N prompt, it would print out the string "<developer email alias> helped with the new DOS, Microsoft Rules!"
To my knowledge, nobody ever figured out how to get access to this particular easter egg, although I do remember Peter Norton writing a column about it in PC-WEEK (he found the text of the easter egg by running "strings" on the recover.com binary).
Nowadays, adding an easter egg to a Microsoft OS is immediate grounds for termination, so it's highly unlikely you'll ever see another.
Somewhat later: I dug up the documentation for the "recover" command - the version of the documentation I found indicates that the tool was intended to recover files with bad sectors within it - apparently if you specified a filename, it would create a new file in the current directory that contained all the clusters from the bad file that were readable. If you specified just a drive, it did the same thing to all the files on the drive - which had the effect of wiping your entire disk. So the tool isn't TOTALLY stupid, but it still was pretty surprising to me when I stumbled onto it on my test machine one day.