Linux /lee'nuhks/ or /li'nuks/, not /li:'nuhks/ n.
The free Unix workalike created by Linus Torvalds and friends starting about 1991 (the pronunciation /lee'nuhks/ is preferred because the name `Linus' has an /ee/ sound in Swedish). This may be the most remarkable hacker project in history -- an entire clone of Unix for 386, 486 and Pentium micros, distributed for free with sources over the net (ports to Alpha and Sparc and many other machines are also in use).
Linux is what GNU aimed to be, and it relies on the GNU toolset. But the Free Software Foundation didn't produce the kernel to go with that toolset until 1999, which was too late. Other, similar efforts like FreeBSD and NetBSD have been technically successful but never caught fire the way Linux has; as this is written in 1999, Linux is seriously challenging Microsoft's OS dominance.
An earlier version of this entry opined "The secret of Linux's success seems to be that Linus worked much harder early on to keep the development process open and recruit other hackers, creating a snowball effect." Truer than we knew. See bazaar.
(A few people object that the name `Linux' should be used to refer only to the kernel, not the entire operating system. This is at best pedantry and at worst axe-grinding; the agenda behind it is usually to claim that the FSF should get most of the credit for Linux because RMS and friends wrote many of its user-level tools.)