Gains by Linux are old (if not Red) hat by now. Still, it's gratifying to see the numbers. Here are the latest (April 1999) from The Internet Operating System Counter (http://leb.net/hzo/):
Host OS recognized (grouped, sorted) 01/99 %recog 04/99 %recog +/-%
------------------------------------ ------- ------ ------- ------- ----
1.( ) Linux 287093 28.5 399748 31.3 +2.8
2.( ) Windows 95/98/NT 245184 24.4 310162 24.3 -0.1
3.( ) Solaris/SunOS 178350 17.7 213304 16.7 -1.0
4.( ) BSD Family 150961 15.0 186385 14.6 -0.4
5.( ) IRIX 52941 5.3 58112 4.6 -0.7
6.( 7.) Mac/Apple 16170 1.6 26548 2.1 +0.5
7.( 6.) AIX 17362 1.7 18427 1.4 -0.3
8.( 9.) HPUX 10416 1.0 11760 0.9 -0.1
9.( 8.) Reliant Unix/Sinix 12848 1.3 10616 0.8 -0.5
10.(12.) Novell Netware 3656 0.4 5982 0.5 +0.1
11.(10.) Digital Unix 5016 0.5 5186 0.4 -0.1
12.(11.) SCO Unix 3787 0.4 4021 0.3 -0.1
----------- ----- ----- ------- -----
++checksums 983784 97.8 1250250 97.9
Here's the graphical view —
Source: The Internet Operating System Counter