The Kindle: powered by Linux
The power of free, open-source software is everywhere. Even in Amazon's new Kindle e-book reader:
Linux on Amazon's Kindle e-book reader
Kindle powered by Linux
In keeping with the GPL license that governs GNU/Linux, anybody modifying the code must, in turn release its source code, as Amazon has done here.
Other huge things that use Linux: Google's new Android cell-phone operating system, and our beloved TiVo.
And you might not know that Apple's OS X is powered by BSD, a descendant of Unix. The licenses that govern the various BSD distributions are different than those for Linux, and companies that modify BSD are not bound to release their source code. Hence, OS X is not open-source. Perhaps the ability to keep the system closes influenced Apple's decision to go with BSD instead of Linux when it was developing OS X. I say "perhaps" because I have no idea.