Linux has continued this growth in 1999, and 25% of server operating systems shipped in last year were Linux. (It's interesting to note that 1999 was the first year NT failed to grow its market share.)
Another report has Linux use increasing by 25% each year for the next four years.
For some information on the recent history of Linux, please see the 1998 Linux timeline at the Linux Weekly News.
It's worth noting that, currently, overall Windows usage dwarfs Linux usage by a large margin (perhaps 100 to one or more). However, in the high-end market, where Microsoft's Windows NT / "Windows 2000" competes, Linux and Windows are far closer.
It's also worth noting the reasons cited for NT's growth...