Linux

Linux: n.  The free UNIX workalike created by Linus Torvalds
   and friends starting about 1990.  This may be the most remarkable
   hacker project in history -- an entire clone of UNIX for 386 and
   486 micros, distributed for free with sources over the net.  This
   is what {GNU} aimed to be, but the Free Software Foundation
   never produced the kernel to go with its UNIX toolset (which Linux
   uses).  Other, similar efforts like FreeBSD and NetBSD have been
   much less successful.  The secret of Linux's success may be that
   Linus worked much harder early on to keep the development process
   open and recruit other hackers, creating a snowball effect.