iMac secret feature or application?
If it looked like this:



you just entered Time Machine. From Apple:


  Time Machine is the built-in backup that works with your Mac and an external drive (sold separately) or Time Capsule. Connect the drive, assign it to Time Machine, and start enjoying some peace of mind. Time Machine automatically backs up your entire Mac, including system files, applications, accounts, preferences, music, photos, movies, and documents. But what makes Time Machine different from other backup applications is that it not only keeps a spare copy of every file, it remembers how your system looked on any given day—so you can revisit your Mac as it appeared in the past.


It's not a hidden feature but it does aim to stay unobtrusive and out-of-the-way until you need it.

You can get to Time Machine any time you want by clicking this icon in your menu bar and selecting Enter Time Machine.



If you don't see the Time Machine icon in your menu bar go to System Preferences -> Time Machine and make sure the Show Time Machine status in menu bar option is checked.