Why Red Hat 7.3 is (still) so present on servers
Red Hat 7.3 came out in the summer of 2002, but much to various people's horror you can still find it on servers (along with Red Hat 7.2, another perennial favorite). I think that there are at least two reasons for this, and they work together in combination.
First, Red Hat 7.3 was the last Red Hat release until Fedora Core 2 where you could run a stock kernel. From Red Hat 8 through FC 1, you needed a 2.4 kernel with the NPTL patches, which meant that you had to use a Red Hat kernel, and in turn this probably forced people into upgrading machines running those later distributions as kernel security issues turned up. On RH 7.3, you could just build your own kernel in response to problems.
(Our remaining 7.3 machines are not running Red Hat kernels, for example; they're running relatively current 2.4 kernels we built ourselves.)
As part of this, RH 8 and RH 9 were at least seen as much more experimental distributions than RH 7.3. Red Hat 7.3 enjoyed a reputation of stability and quality which made it a good choice for several years, partly because it was seen as a point release in the 7.x series instead of a new 'major' version.
(For example, our 7.3 machines were installed in mid to late 2003, but it was clearly still seen as a good choice even then, despite Red Hat 9 having been released.)
Second, I think that Red Hat 7.3 just arrived at the right time, when people were increasingly willing to install Linux and Linux was looking increasingly like a good choice. And systems often don't get upgraded if they keep on working, so the first serious wave of adoption can stick around for quite a while.
(The 2.4 kernel series had a lot of good stuff going for it by 7.3, including stability, decent NFS, good SMP support, and ext3. I can't remember if it had XFS, but I believe SGI was at least working on XFS at the time, which served as a useful endorsement.)
(This entry was sparked by Dave Jones' entry about running into obsolete Red Hat kernels.)