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 2001, Linux is seriously challenging Microsoft's OS dominance. It has already captured 31% of the Internet-server market and 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).