The list of distros listed below are barely making more than 5% of the Linux market share by summing all of them (let alone the global OS market). However, in Greece we have a saying: "Greece is not just Athens". Similarly, 'Linux' is not just Red Hat. So the following distros are interesting to watch out their evolution, see which come and go, which stay and become more popular with time! Let's have a look:
In the footsteps of the big three desktop-oriented distros, you will find Yoper, Ark Linux, Vector Linux, Libranet, ELX.
Yoper and Vector Linux had new releases very recently and they seem to be the strongest in this category, with the most potential. Ark Linux is still in alpha, but it is getting there bit by bit.
The Debian-based Libranet had a release a few months ago, but the Canadian company behind it keeps a pretty low profile, so not much is known about their plans. ELX is an effort of two people mostly, it started with a lot of fanfare, but it seems this to have died out recently.
Knoppix is the "king" of the CD-based distros. You pop-in its bootable CD and no installation will be required. It is great for demos. MoviX is getting momentum as well recently in this area.
Peanut Linux is a mini-distro. Only 340 MBs of download, a good choice for people with not fast internet connection.
Yellow Dog Linux is the main distro in the PPC architecture. While Debian, SuSE and Mandrake make PPC releases every blue moon, YDL is the one really tested with many PPC machines, and it is geared directly from PPC people to PPC users. Its only focus is the PPC, it has a very nice installation system that will help Mac users add YDL to their configuration easily etc.
Connectiva is the leading distro for the latin-speaking Linux userbase. They are based in Brasil.
CRUX is really the hobby of a Swedish guy. However, its lean i686 optimized distro has gained a lot of fans. On the same theme, ArchLinux and LinuxInstall are also interesting and maybe IcePack too.
The main source-based distro today is of course Gentoo. If you are a friend of source-based distros (meaning that almost everything needs to be recompiled by the user) you might want to have a look at Sorcerer, Lunar and SourceMage.
In the server market, TurboLinux was always a respectable player, but after the buyout of the company recently, the future of this distro is not certain.
United Linux is the common effort of SuSE, Connectiva and SCO, and its main goal is to become the No 2 server Linux market power behind Red Hat. They are doing good so far. SuSE is the main player behind this specific initiative.
SCO/Caldera used to be one of the most important players in the Linux distro market a few years ago, but since Caldera acquired SCO, a lot of things have changed in their strategies. Someone could argue that they are not even "Linux-friendly" anymore.
To get more information about the "other" Linux distros and always stay in touch with the latest happenings, you can always visit DistroWatch, a great resource for all distros, including the very small ones.