The free Unix workalike created by Linus Torvalds and friends starting about 1991. The pronunciation /liŽnuhks/ is preferred because the name ‘Linus’ has an /ee/ sound in Swedish (Linus's family is part of Finland's 6% ethnic-Swedish minority) and Linus considers English short /i/ to be closer to /ee/ than English long /i:/. 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 2003, Linux has effectively swallowed all proprietary Unixes except Solaris and is seriously challenging Microsoft. It has already captured 41% of the Internet-server market and over 25% of general business servers.
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.
(Some people object that the name ‘Linux’ should be used to refer only to the kernel, not the entire operating system. This claim is a proxy for an underlying territorial dispute; people who insist on the term GNU/Linux want the FSF to get most of the credit for Linux because RMS and friends wrote many of its user-level tools. Neither this theory nor the term GNU/Linux has gained more than minority acceptance).