Like petty officers on a ship, the distributions in this category represent a parallel hierarchy. In other words, while there is a continuity in the rankings of the first and second tier, those in the third tier have settled down to become prominent in their own niches. Unlike those in the first and second tier, these distributions make no attempt to offer something for everyone; instead, they are designed for very specific purposes. For this reason, distros in this category are judged less by popularity or influence than by how useful they are for particular purposes.
The truth is, there are too many specialized distros to list them all. However, here are some of the better known ones:
Damn Small Linux (DSL): One of the first and best-known distributions for small or minimal systems, DSL has developed a community that is unusually loyal for free software. Among its peculiarities is a continued use of the 2.4 kernel and a resolution to limit the size to 50 megabytes.
Foresight: Designed for those who want the cutting edge of GNOME and of cross-desktop tools, Foresight is one of the few distributions to use the Conary packaging system, which combines modern packaging techniques with version control. Although Conary has so far failed to overtake RPMs and DEBs, many of those who have used it regard Conary as the future of package management.
K12LTSP: As the mouthful of a name suggests, K12LTSP is doubly a specialist: It is an educational distro (K-12) designed for use on thin clients (LTSP). Within this space, it has few rivals.
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Knoppix: One of the many Debian-derived distros, Knoppix was the first live CD to capture the community's attention. Although it is far from being alone today, it remains many user's choice of rescue disks.
Puppy Linux: Like DSL, Puppy Linux is a minimalist distribution. Built from scratch rather than derived from another distro, it runs off a RAM disk and uses smaller window managers like Fluxbox and IceWM for a graphical interface.The limits of ranking
Nothing in this division suggests technical or philosophical superiority -- just popularity and influence. Rather than an exact taxonomy, these rankings should be thought of as an attempt to summarize the reactions to various distributions. So far as possible, I have tried to observe why each distribution is regarded in the way that it is rather than voice my own views, although undoubtedly my own bias has crept into some of the descriptions.
Nor have I tried to categorize every distribution known to me. For one thing, I haven't tried every known distribution, and would have no wish to devote the rest of my life to doing so.
For another, aside from the specializations that clearly fall into the third tier, beyond the three categories I have mentioned, rankings become harder to describe. For every distribution like Ubuntu that quickly establishes its position at the head of the pack, dozens more exist, whose popularity is more like that of a penny mining stock, changing dramatically from year to year.
Many, such as Zenwalk or Dreamlinux are too recent for their position to be clear for another year or two, while others never seem to have been intended as anything more than a two- or three-person project.
And the ranks within each tier? That is something I have no intention of getting into. Although how each distribution is regarded broadly speaking is relatively easy to observe, beyond that, personal experience and enthusiasm create too much static for consistently clear signals.
All the same, placing distributions into general categories can be useful if you're browsing distros, especially if you are new. Noting that a distro is popular or influential is worthwhile, but knowing the reasons for its popularity or how it has influenced is better still. If nothing else, this knowledge can save you time by allowing you to target what interests you.